R ggplot: Move y axis to grid lines on polar plot (Polar_Coord)

Question

I'm creating a polar plot showing a histogram of the direction travlled of paired group data. More specifically, directions travelled between different groups of sibling.

Here is a mock:

mockdf <- data.frame(dir = as.numeric( runif( 1000, -pi/2, pi) ),
                 ID = sample(letters[1:2], 1000, TRUE))
ggplot(data=mockdf, aes(x=mockdf$dir)) +
  coord_polar(theta = "x", start = pi, direction = 1) +
  scale_fill_manual(name = "Sibling", values=c("black", "White")) +
  geom_histogram(bins=32, aes(fill=mockdf$ID), color= "black") +
  facet_wrap(~mockdf$ID) +
  scale_y_continuous("Number of reloactions", limits = c(-8,30)) +
  scale_x_continuous(limits = c(-pi,pi), breaks = c(0, pi/4, pi/2, 3*pi/4, 
  pi, -3*pi/4, -pi/2, -pi/4),
                     labels = c("N", "NE", "E", "SE", "S", "SW", "W", "NW"))

I'd like to move the y axis labels of 0, 10, 20, 30 onto the grid itself (i.e. along the SW orientation), but having difficulties doing so. Does anyone know how I go about doing this?

Answer 1

You can use options within theme to remove current axis text and tick marks andgeom_text() to manually add the labels. I've demonstrated how to add the first one.

ggplot(data=mockdf, aes(x=mockdf$dir)) +
  coord_polar(theta = "x", start = pi, direction = 1) +
  scale_fill_manual(name = "Sibling", values=c("black", "White")) +
  geom_histogram(bins=32, aes(fill=mockdf$ID), color= "black") +
  facet_wrap(~mockdf$ID) +
  scale_y_continuous("Number of reloactions", limits = c(-8,30)) +
  scale_x_continuous(limits = c(-pi,pi), breaks = c(0, pi/4, pi/2, 3*pi/4, 
                                                pi, -3*pi/4, -pi/2, -pi/4),
                 labels = c("N", "NE", "E", "SE", "S", "SW", "W", "NW")) + 
  # remove text and tick marks from axis
   theme(axis.text.y = element_blank(),
         axis.ticks.y = element_blank()) + 
  # add label manually
  geom_text(x = 4, y = 10, label = "10")