ЦИЯ:

каждыйnode должен иметь соответствующий флаг, это не так.

КОД:

body {
    font-family: Lato;
    background-color: #F5F5F5;
}

.title {
    margin-top: 30px;
    margin-bottom: 30px;
}

#results {
    text-align: center;
}

#results div {
    margin-top: 15px;
}

#results {
    background-color: white;
    margin: auto;
    width: 1200px;
    padding: 40px;
    box-shadow: 2px 2px 2px black;
}

/*!
 * Generated with CSS Flag Sprite generator (https://www.flag-sprites.com/)
 */

.flag {
    display: inline-block;
    width: 16px;
    height: 11px;
    background-image: url("https://image.ibb.co/gMUNBQ/flags.png") no-repeat;
}


.flag.flag-td {
    background-position: -240px -132px;
}

.flag.flag-pw {
    background-position: 0 -121px;
}

.flag.flag-fk {
    background-position: -32px -44px;
}

.flag.flag-im {
    background-position: -48px -66px;
}

.flag.flag-fr {
    background-position: -80px -44px;
}

.flag.flag-sh {
    background-position: -240px -121px;
}

.flag.flag-bg {
    background-position: -64px -11px;
}

.flag.flag-ly {
    background-position: -48px -88px;
}

.flag.flag-tk {
    background-position: -64px -143px;
}

.flag.flag-re {
    background-position: -48px -121px;
}

.flag.flag-nl {
    background-position: -240px -99px;
}

.flag.flag-sv {
    background-position: -160px -132px;
}

.flag.flag-sa {
    background-position: -128px -121px;
}

.flag.flag-ck {
    background-position: -128px -22px;
}

.flag.flag-tr {
    background-position: -144px -143px;
}

.flag.flag-pr {
    background-position: -224px -110px;
}

.flag.flag-tv {
    background-position: -176px -143px;
}

.flag.flag-sd {
    background-position: -192px -121px;
}

.flag.flag-uy {
    background-position: -16px -154px;
}

.flag.flag-bb {
    background-position: 0 -11px;
}

.flag.flag-za {
    background-position: -256px -154px;
}

.flag.flag-vu {
    background-position: -144px -154px;
}

.flag.flag-ba {
    background-position: -256px 0;
}

.flag.flag-kr {
    background-position: -80px -77px;
}

.flag.flag-tf {
    background-position: -256px -132px;
}

.flag.flag-cl {
    background-position: -144px -22px;
}

.flag.flag-ki {
    background-position: -16px -77px;
}

.flag.flag-lr {
    background-position: -240px -77px;
}

.flag.flag-do {
    background-position: -96px -33px;
}

.flag.flag-dz {
    background-position: -112px -33px;
}

.flag.flag-cr {
    background-position: -208px -22px;
}

.flag.flag-ar {
    background-position: -144px 0;
}

.flag.flag-sb {
    background-position: -144px -121px;
}

.flag.flag-tj {
    background-position: -48px -143px;
}

.flag.flag-cw {
    background-position: -256px -22px;
}

.flag.flag-bv {
    background-position: -224px -11px;
}

.flag.flag-pn {
    background-position: -208px -110px;
}

.flag.flag-so {
    background-position: -80px -132px;
}

.flag.flag-la {
    background-position: -160px -77px;
}

.flag.flag-me {
    background-position: -112px -88px;
}

.flag.flag-sx {
    background-position: -176px -132px;
}

.flag.flag-sy {
    background-position: -192px -132px;
}

.flag.flag-gq {
    background-position: -32px -55px;
}

.flag.flag-ro {
    background-position: -64px -121px;
}

.flag.flag-kurdistan {
    background-position: -96px -77px;
}

.flag.flag-si {
    background-position: -256px -121px;
}

.flag.flag-mh {
    background-position: -144px -88px;
}

.flag.flag-ky {
    background-position: -128px -77px;
}

.flag.flag-eh {
    background-position: -176px -33px;
}

.flag.flag-ru {
    background-position: -96px -121px;
}

.flag.flag-tz {
    background-position: -208px -143px;
}

.flag.flag-np {
    background-position: 0 -110px;
}

.flag.flag-pg {
    background-position: -128px -110px;
}

.flag.flag-lb {
    background-position: -176px -77px;
}

.flag.flag-sn {
    background-position: -64px -132px;
}

.flag.flag-gw {
    background-position: -112px -55px;
}

.flag.flag-tt {
    background-position: -160px -143px;
}

.flag.flag-eg {
    background-position: -160px -33px;
}

.flag.flag-gn {
    background-position: 0 -55px;
}

.flag.flag-al {
    background-position: -80px 0;
}

.flag.flag-jm {
    background-position: -192px -66px;
}

.flag.flag-be {
    background-position: -32px -11px;
}

.flag.flag-kn {
    background-position: -48px -77px;
}

.flag.flag-mu {
    background-position: -48px -99px;
}

.flag.flag-az {
    background-position: -240px 0;
}

.flag.flag-gh {
    background-position: -208px -44px;
}

.flag.flag-vg {
    background-position: -96px -154px;
}

.flag.flag-ug {
    background-position: -240px -143px;
}

.flag.flag-rw {
    background-position: -112px -121px;
}

.flag.flag-pl {
    background-position: -176px -110px;
}

.flag.flag-at {
    background-position: -176px 0;
}

.flag.flag-bs {
    background-position: -192px -11px;
}

.flag.flag-ee {
    background-position: -144px -33px;
}

.flag.flag-no {
    background-position: -256px -99px;
}

.flag.flag-ca {
    background-position: -16px -22px;
}

.flag.flag-ml {
    background-position: -176px -88px;
}

.flag.flag-vc {
    background-position: -64px -154px;
}

.flag.flag-mc {
    background-position: -80px -88px;
}

.flag.flag-vi {
    background-position: -112px -154px;
}

.flag.flag-fj {
    background-position: -16px -44px;
}

.flag.flag-sj {
    background-position: 0 -132px;
}

.flag.flag-ic {
    background-position: -240px -55px;
}

.flag.flag-fo {
    background-position: -64px -44px;
}

.flag.flag-lu {
    background-position: -16px -88px;
}

.flag.flag-nf {
    background-position: -192px -99px;
}

.flag.flag-mm {
    background-position: -192px -88px;
}

.flag.flag-er {
    background-position: -208px -33px;
}

.flag.flag-somaliland {
    background-position: -96px -132px;
}

.flag.flag-lc {
    background-position: -192px -77px;
}

.flag.flag-cd {
    background-position: -48px -22px;
}

.flag.flag-ma {
    background-position: -64px -88px;
}

.flag.flag-mp {
    background-position: -240px -88px;
}

.flag.flag-mx {
    background-position: -96px -99px;
}

.flag.flag-na {
    background-position: -144px -99px;
}

.flag.flag-gs {
    background-position: -64px -55px;
}

.flag.flag-tw {
    background-position: -192px -143px;
}

.flag.flag-ae {
    background-position: -16px 0;
}

.flag.flag-cu {
    background-position: -224px -22px;
}

.flag.flag-id {
    background-position: -256px -55px;
}

.flag.flag-ht {
    background-position: -208px -55px;
}

.flag.flag-kw {
    background-position: -112px -77px;
}

.flag.flag-by {
    background-position: -256px -11px;
}

.flag.flag-il {
    background-position: -16px -66px;
}

.flag.flag-ps {
    background-position: -240px -110px;
}

.flag.flag-tl {
    background-position: -80px -143px;
}

.flag.flag-ai {
    background-position: -64px 0;
}

.flag.flag-gp {
    background-position: -16px -55px;
}

.flag.flag-pt {
    background-position: -256px -110px;
}

.flag.flag-gr {
    background-position: -48px -55px;
}

.flag.flag-bw {
    background-position: -240px -11px;
}

.flag.flag-kp {
    background-position: -64px -77px;
}

.flag.flag-mq {
    background-position: -256px -88px;
}

.flag.flag-ad {
    background-position: 0 0;
}

.flag.flag-gu {
    background-position: -96px -55px;
}

.flag.flag-cf {
    background-position: -64px -22px;
}

.flag.flag-ni {
    background-position: -224px -99px;
}

.flag.flag-ke {
    background-position: -240px -66px;
}

.flag.flag-mz {
    background-position: -128px -99px;
}

.flag.flag-kh {
    background-position: 0 -77px;
}

.flag.flag-us {
    background-position: 0 -154px;
}

.flag.flag-wf {
    background-position: -176px -154px;
}

.flag.flag-hk {
    background-position: -144px -55px;
}

.flag.flag-ph {
    background-position: -144px -110px;
}

.flag.flag-de {
    background-position: -32px -33px;
}

.flag.flag-sc {
    background-position: -160px -121px;
}

.flag.flag-tibet {
    background-position: -32px -143px;
}

.flag.flag-cy {
    background-position: 0 -33px;
}

.flag.flag-scotland {
    background-position: -176px -121px;
}

.flag.flag-ws {
    background-position: -192px -154px;
}

.flag.flag-ci {
    background-position: -112px -22px;
}

.flag.flag-bf {
    background-position: -48px -11px;
}

.flag.flag-tn {
    background-position: -112px -143px;
}

.flag.flag-my {
    background-position: -112px -99px;
}

.flag.flag-mw {
    background-position: -80px -99px;
}

.flag.flag-an {
    background-position: -112px 0;
}

.flag.flag-nr {
    background-position: -16px -110px;
}

.flag.flag-pm {
    background-position: -192px -110px;
}

.flag.flag-xk {
    background-position: -208px -154px;
}

.flag.flag-es {
    background-position: -224px -33px;
}

.flag.flag-pa {
    background-position: -80px -110px;
}

.flag.flag-jp {
    background-position: -224px -66px;
}

.flag.flag-mr {
    background-position: 0 -99px;
}

.flag.flag-pf {
    background-position: -112px -110px;
}

.flag.flag-cn {
    background-position: -176px -22px;
}

.flag.flag-au {
    background-position: -192px 0;
}

.flag.flag-cv {
    background-position: -240px -22px;
}

.flag.flag-ao {
    background-position: -128px 0;
}

.flag.flag-fi {
    background-position: 0 -44px;
}

.flag.flag-nc {
    background-position: -160px -99px;
}

.flag.flag-ag {
    background-position: -48px 0;
}

.flag.flag-gy {
    background-position: -128px -55px;
}

.flag.flag-nu {
    background-position: -32px -110px;
}

.flag.flag-to {
    background-position: -128px -143px;
}

.flag.flag-qa {
    background-position: -32px -121px;
}

.flag.flag-dm {
    background-position: -80px -33px;
}

.flag.flag-aw {
    background-position: -208px 0;
}

.flag.flag-cz {
    background-position: -16px -33px;
}

.flag.flag-mk {
    background-position: -160px -88px;
}

.flag.flag-ax {
    background-position: -224px 0;
}

.flag.flag-pk {
    background-position: -160px -110px;
}

.flag.flag-st {
    background-position: -144px -132px;
}

.flag.flag-bz {
    background-position: 0 -22px;
}

.flag.flag-br {
    background-position: -176px -11px;
}

.flag.flag-am {
    background-position: -96px 0;
}

.flag.flag-bh {
    background-position: -80px -11px;
}

.flag.flag-gg {
    background-position: -192px -44px;
}

.flag.flag-um {
    background-position: -256px -143px;
}

.flag.flag-ga {
    background-position: -96px -44px;
}

.flag.flag-yt {
    background-position: -240px -154px;
}

.flag.flag-gd {
    background-position: -128px -44px;
}

.flag.flag-dj {
    background-position: -48px -33px;
}

.flag.flag-ss {
    background-position: -128px -132px;
}

.flag.flag-gt {
    background-position: -80px -55px;
}

.flag.flag-sk {
    background-position: -16px -132px;
}

.flag.flag-sm {
    background-position: -48px -132px;
}

.flag.flag-bd {
    background-position: -16px -11px;
}

.flag.flag-jo {
    background-position: -208px -66px;
}

.flag.flag-iq {
    background-position: -96px -66px;
}

.flag.flag-bt {
    background-position: -208px -11px;
}

.flag.flag-it {
    background-position: -144px -66px;
}

.flag.flag-mg {
    background-position: -128px -88px;
}

.flag.flag-sr {
    background-position: -112px -132px;
}

.flag.flag-sg {
    background-position: -224px -121px;
}

.flag.flag-ec {
    background-position: -128px -33px;
}

.flag.flag-ge {
    background-position: -144px -44px;
}

.flag.flag-ie {
    background-position: 0 -66px;
}

.flag.flag-kz {
    background-position: -144px -77px;
}

.flag.flag-tc {
    background-position: -224px -132px;
}

.flag.flag-tg {
    background-position: 0 -143px;
}

.flag.flag-lv {
    background-position: -32px -88px;
}

.flag.flag-mo {
    background-position: -224px -88px;
}

.flag.flag-mv {
    background-position: -64px -99px;
}

.flag.flag-sz {
    background-position: -208px -132px;
}

.flag.flag-va {
    background-position: -48px -154px;
}

.flag.flag-bn {
    background-position: -144px -11px;
}

.flag.flag-bj {
    background-position: -112px -11px;
}

.flag.flag-zm {
    background-position: -16px -165px;
}

.flag.flag-co {
    background-position: -192px -22px;
}

.flag.flag-mn {
    background-position: -208px -88px;
}

.flag.flag-zw {
    background-position: -32px -165px;
}

.flag.flag-vn {
    background-position: -128px -154px;
}

.flag.flag-sl {
    background-position: -32px -132px;
}

.flag.flag-gi {
    background-position: -224px -44px;
}

.flag.flag-as {
    background-position: -160px 0;
}

.flag.flag-catalonia {
    background-position: -32px -22px;
}

.flag.flag-gb {
    background-position: -112px -44px;
}

.flag.flag-gl {
    background-position: -240px -44px;
}

.flag.flag-england {
    background-position: -192px -33px;
}

.flag.flag-py {
    background-position: -16px -121px;
}

.flag.flag-th {
    background-position: -16px -143px;
}

.flag.flag-eu {
    background-position: -256px -33px;
}

.flag.flag-zanzibar {
    background-position: 0 -165px;
}

.flag.flag-ch {
    background-position: -96px -22px;
}

.flag.flag-km {
    background-position: -32px -77px;
}

.flag.flag-om {
    background-position: -64px -110px;
}

.flag.flag-et {
    background-position: -240px -33px;
}

.flag.flag-fm {
    background-position: -48px -44px;
}

.flag.flag-ve {
    background-position: -80px -154px;
}

.flag.flag-io {
    background-position: -80px -66px;
}

.flag.flag-bm {
    background-position: -128px -11px;
}

.flag.flag-lk {
    background-position: -224px -77px;
}

.flag.flag-ye {
    background-position: -224px -154px;
}

.flag.flag-ms {
    background-position: -16px -99px;
}

.flag.flag-is {
    background-position: -128px -66px;
}

.flag.flag-hu {
    background-position: -224px -55px;
}

.flag.flag-rs {
    background-position: -80px -121px;
}

.flag.flag-md {
    background-position: -96px -88px;
}

.flag.flag-hn {
    background-position: -176px -55px;
}

.flag.flag-ne {
    background-position: -176px -99px;
}

.flag.flag-mt {
    background-position: -32px -99px;
}

.flag.flag-ng {
    background-position: -208px -99px;
}

.flag.flag-kg {
    background-position: -256px -66px;
}

.flag.flag-je {
    background-position: -176px -66px;
}

.flag.flag-in {
    background-position: -64px -66px;
}

.flag.flag-hr {
    background-position: -192px -55px;
}

.flag.flag-hm {
    background-position: -160px -55px;
}

.flag.flag-cm {
    background-position: -160px -22px;
}

.flag.flag-uz {
    background-position: -32px -154px;
}

.flag.flag-wales {
    background-position: -160px -154px;
}

.flag.flag-gm {
    background-position: -256px -44px;
}

.flag.flag-li {
    background-position: -208px -77px;
}

.flag.flag-nz {
    background-position: -48px -110px;
}

.flag.flag-bo {
    background-position: -160px -11px;
}

.flag.flag-se {
    background-position: -208px -121px;
}

.flag.flag-tm {
    background-position: -96px -143px;
}

.flag.flag-cg {
    background-position: -80px -22px;
}

.flag.flag-ir {
    background-position: -112px -66px;
}

.flag.flag-dk {
    background-position: -64px -33px;
}

.flag.flag-gf {
    background-position: -160px -44px;
}

.flag.flag-bi {
    background-position: -96px -11px;
}

.flag.flag-pe {
    background-position: -96px -110px;
}

.flag.flag-ua {
    background-position: -224px -143px;
}

.flag.flag-af {
    background-position: -32px 0;
}

.flag.flag-ls {
    background-position: -256px -77px;
}

.flag.flag-lt {
    background-position: 0 -88px;
}

    <html>
    <head>
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" integrity="sha384-rwoIResjU2yc3z8GV/NPeZWAv56rSmLldC3R/AZzGRnGxQQKnKkoFVhFQhNUwEyJ" crossorigin="anonymous">
        <link href="https://fonts.googleapis.com/css?family=Lato" rel="stylesheet">
        <link rel="stylesheet" href="main.css">
        <style>
            body { 
                font-family: Lato;
                background-color: #F5F5F5; 
            }
        </style>
    </head>
    <body>
        <div class="body">
            <div class="container-fluid">
                <h1 class="text-center title">Countries linked by Border</h1>
                <div class="text-center">
                    <div id="results"></div>
                    <div id="flags"></div>
                </div>
            </div>
        </div>
    </body>
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script
  src="https://code.jquery.com/jquery-3.1.1.min.js"
  integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8="
  crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js" integrity="sha384-DztdAPBWPRXSA/3eYEEUWrWCy7G5KFbe8fFjk5JAIxUYHKkDx6Qin1DkWx51bBrb" crossorigin="anonymous"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js" integrity="sha384-vBWWzlZJ8ea9aCX4pEW3rVHjgjt7zpkNpZk+02D9phzyeVkE+jo0ieGizqPLForn" crossorigin="anonymous"></script>
    
   <script type="text/javascript">
       
       // Acknowledgements:
       
       // Excelent D3 V4 tutorial: http://www.puzzlr.org/force-graphs-with-d3/
       
       // Drag Example: https://github.com/jenovs/chingu-fcc-speedrun-challenge/blob/master/data-vis/force-directed-graph/src/components/ForceGraph/ForceGraph.js
       
       
    d3.json("https://raw.githubusercontent.com/DealPete/forceDirected/master/countries.json", function(error, json) {

          if (error) {
            return console.warn(error);
          }

          const data = json;
          const nodes = data.nodes;
          const links = data.links;
     
            const margin = {
              top: 0,
              right: 0,
              bottom: 0,
              left: 0
            }

            const w = 1250 - margin.left - margin.right;
            const h = 550 - margin.top - margin.bottom;
        
            const results = d3.select('#results')

            const svg = results.append("svg")
              .attr("width", w + margin.left + margin.right)
              .attr("height", h + margin.top + margin.bottom);

            const simulation = d3.forceSimulation()
                .nodes(nodes)
                .force("charge_force", d3.forceManyBody().strength(-3))
                .force("center_force", d3.forceCenter(w / 2, h / 2));
        
            const div = d3.select("body")
                .append("div")	
                .attr("class", "tooltip")				
                .style("opacity", 0);

            const link = svg.append("g")
              .attr("class", "links")
              .selectAll("line")
              .data(links)
              .enter().append("line")
              .attr('stroke', 'black')
              .attr("stroke-width", 2);

            const node = results.select('#flags').selectAll('.node')
                .data(data.nodes)
                .enter()
                .append('img')
                .attr('class', d => 'flag flag-' + d.code)
                .on("mouseover", function(d) {		
                div.transition()		
                    .duration(200)		
                    .style("opacity", .9);		
                div	.html(d.country)	
                    .style("left", (d3.event.pageX - 7) + "px")		
                    .style("top", (d3.event.pageY - 30) + "px");	
                })					
                .on("mouseout", function(d) {		
                div.transition()		
                    .duration(500)		
                    .style("opacity", 0);	
                }).call(d3.drag()
                .on('start', dragStarted)
                .on('drag', dragged)
                .on('end', dragEnded));

            const link_force = d3.forceLink(links)

            simulation.force("links", link_force)

            simulation.on("tick", tickActions);

            function tickActions() {
              node
                .attr("transform", function(d) {
                  return "translate(" + d.x + "," + d.y + ")"
                });

              link
                .attr("x1", function(d) {
                  return d.source.x;
                })
                .attr("y1", function(d) {
                  return d.source.y;
                })
                .attr("x2", function(d) {
                  return d.target.x;
                })
                .attr("y2", function(d) {
                  return d.target.y;
                });
            }
        
            function dragStarted(d) {
                if (!d3.event.active) sim.alphaTarget(0.3).restart();
                d.fx = d.x;
                d.fy = d.y;
            }

            function dragged(d) {
                d.fx = d3.event.x;
                d.fy = d3.event.y;
            }

            function dragEnded(d) {
                if (!d3.event.active) sim.alphaTarget(0);
                d.fx = null;
                d.fy = null;
            }

        });
  </script>

</html>