options(repr.plot.width=7, repr.plot.height=6)
# facet boxplot
bp <- ggplot(expr_data2, aes(x=group, y=expression, fill=NA)) + 
  geom_boxplot(outlier.size=NA, size=0.01, outlier.shape = NA) + 
  geom_jitter(width = 0.3, size=0.01, aes(color=cluster)) +
  # + geom_boxplot( + 
  facet_grid( cluster ~ gene, switch="y") + # , scales = "free"
  theme_bw() + 
  stat_compare_means(aes(group = group, label = ..p.signif..), label.x = 1.3,label.y = 1.3, 
                     method = "wilcox.test", hide.ns = T) + # label = "p.format",
  theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank()) +
  labs(x = "", y = "", title = "") +
  theme(panel.spacing=unit(.3, "lines"),panel.border = element_rect(color = "black", fill = NA, size = 0.2)) +
  theme(axis.ticks.x = element_blank(), axis.ticks = element_line(size = 0.1), 
        axis.text.x  = element_text(face="plain", angle=90, size = 8, color = "black", vjust=0.5),
        axis.text.y  = element_text(face="plain", size = 4, color = "black"),
        axis.title =element_text(size = 12)) +
  theme(strip.background = element_rect(fill = "gray97", color = NA))+
  theme(legend.position = "none") +
  theme(strip.placement = "outside", strip.text.x = element_text(face="italic", size = 11),
          strip.text.y = element_text(face="plain", size = 11)) +
  scale_y_continuous(position="right", limits = c(-0.5,1.5)) +
  scale_fill_manual(values=brewer.pal(8,"Set2")[c(2,3,7,1,5,6)]) +
  scale_color_manual(values=brewer.pal(8,"Set2")[c(2,3,7,1,5,6)])
bp