r - Using ggsignif with grouped bar graphs and facet_wrap not working

Question

I am trying to use ggsignif for displaying significant stars in top of paired bar graphs using facet_wrap. However, I can′t manage to find a way of displaying one significant bar per facet. Here is what I mean:

    dat <- data.frame(Group = c("S1", "S1", "S2", "S2"),
                  Sub   = c("A", "B", "A", "B"),
                  Value = c(3,5,7,8))

    ggplot(dat, aes(Group, Value)) +
      geom_bar(aes(fill = Sub), stat="identity", position="dodge", width=.5) +
      geom_signif(y_position=c(5.3, 8.3), xmin=c(0.8, 1.8), xmax=c(1.2, 2.2),
              annotation=c("**", "NS"), tip_length=0) +
      scale_fill_manual(values = c("grey80", "grey20")) +
      facet_grid(~ Group, scales = "free")

Is there a way of making sure that each facet has its individual significance label?

Answer 1

The main problem seems to me is that the geom_signif layer doesn't know to what panel the variables go to, since it has no data argument provided.

I'm not that familiar with the package, but the documentation seems to suggest that manual = TRUE is recommended for plotting it in different facets. Doing that and making some adjustments for the errors that were thrown, I got the following to work:

ggplot(dat, aes(Group, Value)) +
  geom_bar(aes(fill = Sub), stat="identity", position="dodge", width=.5) +
  geom_signif(data = data.frame(Group = c("S1","S2")),
              aes(y_position=c(5.3, 8.3), xmin=c(0.8, 0.8), xmax=c(1.2, 1.2),
              annotations=c("**", "NS")), tip_length=0, manual = T) +
  scale_fill_manual(values = c("grey80", "grey20")) +
  facet_grid(~ Group, scales = "free")

The key seemed to be to provide a data argument from which the facetting code could deduce what bit goes in what panel.