Kiedy uruchamiam bardzo prosty kod za pomocą pydot

import pydot
graph = pydot.Dot(graph_type='graph')

for i in range(3):

  edge = pydot.Edge("king", "lord%d" % i)
  graph.add_edge(edge)

vassal_num = 0
for i in range(3):
  for j in range(2):
    edge = pydot.Edge("lord%d" % i, "vassal%d" % vassal_num)
    graph.add_edge(edge)
    vassal_num += 1

graph.write_png('example1_graph.png')

Wyświetla mi komunikat o błędzie:

Couldn't import dot_parser, loading of dot files will not be possible.

Używam python 2.7.3