构面网格中的标题宽度

Question

是否可以更改构面图中标题的宽度？

这是我的代码：

ggplot(matrix_cpm_spike_norm_iso.all, aes(miRNA,value))   +
  geom_boxplot(aes(fill = Condition)) + 
  facet_grid(~Experiment, space = "free_y", scale="free") + 
  theme_bw(base_size=12) +
  theme(axis.text.x = element_text(colour = "black",angle=90 )) +
  labs(x="miRNA",y="Expression (log2 cpm)")