ggplot2: Como forçar o número de facetas com muito poucos gráficos?

Para plotar 9 histogramas por gráfico ggplot, usei os seguintes dados:

                               id               variable      value
1                     Segment III | RIM BlackBerry Pearl | 5.600000
2                    Display size | RIM BlackBerry Pearl | 6.500000
3              Voice/call quality | RIM BlackBerry Pearl | 5.600000
4  Instant messaging availability | RIM BlackBerry Pearl | 7.200000
5                   Media quality | RIM BlackBerry Pearl | 6.100000
6          Ease of use for typing | RIM BlackBerry Pearl | 5.700000
7        Speed in accessing email | RIM BlackBerry Pearl | 6.400000
8                      Segment II | RIM BlackBerry Pearl | 5.545455
9                 Value for money | RIM BlackBerry Pearl | 6.000000
10                    Segment III |       Palm Treo 700p | 4.320000
11                   Display size |       Palm Treo 700p | 6.500000
12             Voice/call quality |       Palm Treo 700p | 8.000000
13 Instant messaging availability |       Palm Treo 700p | 5.100000
14                  Media quality |       Palm Treo 700p | 7.000000
15         Ease of use for typing |       Palm Treo 700p | 6.200000
16       Speed in accessing email |       Palm Treo 700p | 6.500000
17                     Segment II |       Palm Treo 700p | 4.454545
18                Value for money |       Palm Treo 700p | 5.400000
19                    Segment III |           Motorola Q | 4.680000
20                   Display size |           Motorola Q | 7.400000
21             Voice/call quality |           Motorola Q | 4.800000
22 Instant messaging availability |           Motorola Q | 5.300000
23                  Media quality |           Motorola Q | 6.900000
24         Ease of use for typing |           Motorola Q | 7.400000
25       Speed in accessing email |           Motorola Q | 8.000000
26                     Segment II |           Motorola Q | 3.121212
27                Value for money |           Motorola Q | 4.200000
28                    Segment III |           Nokia 9300 | 4.360000
29                   Display size |           Nokia 9300 | 6.400000
30             Voice/call quality |           Nokia 9300 | 7.800000
31 Instant messaging availability |           Nokia 9300 | 6.700000
32                  Media quality |           Nokia 9300 | 5.900000
33         Ease of use for typing |           Nokia 9300 | 4.500000
34       Speed in accessing email |           Nokia 9300 | 6.300000
35                     Segment II |           Nokia 9300 | 7.181818
36                Value for money |           Nokia 9300 | 4.600000
37                    Segment III |  Sony Ericsson M600i | 4.360000
38                   Display size |  Sony Ericsson M600i | 7.300000
39             Voice/call quality |  Sony Ericsson M600i | 8.000000
40 Instant messaging availability |  Sony Ericsson M600i | 1.500000
41                  Media quality |  Sony Ericsson M600i | 7.800000
42         Ease of use for typing |  Sony Ericsson M600i | 5.000000
43       Speed in accessing email |  Sony Ericsson M600i | 8.100000
44                     Segment II |  Sony Ericsson M600i | 3.606061
45                Value for money |  Sony Ericsson M600i | 4.000000
46                    Segment III |            Sidekick3 | 7.040000
47                   Display size |            Sidekick3 | 7.200000
48             Voice/call quality |            Sidekick3 | 6.300000
49 Instant messaging availability |            Sidekick3 | 7.200000
50                  Media quality |            Sidekick3 | 6.400000
51         Ease of use for typing |            Sidekick3 | 6.800000
52       Speed in accessing email |            Sidekick3 | 6.200000
53                     Segment II |            Sidekick3 | 3.424242
54                Value for money |            Sidekick3 | 5.300000

Então eu usei o seguinte código:

ggplot(data = data_sub, aes(x = variable, y = value)) +
  geom_bar(stat = "identity") +
  facet_wrap(~id, ncol = 3) +
  coord_flip() +
  theme(axis.title.x = element_blank(),
        axis.title.y = element_blank(),
        panel.grid   = element_blank(),
        legend.position = "none")

E pegou :

Minha pergunta :

Quando tenho menos gráficos, por exemplo, apenas um, gostaria de manter essa formatação. No entanto, recebo apenas um grande gráfico como o seguinte (não ligue para as lendas).

Como posso obter algo parecido com o seguinte?

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