hadley/ggplot2 — comment on commit 42c1ed3f74a9acaa15a8d1058ded2217f2908ed5
File:  line 0
By kohske:

Now I remember why I implemented this!

http://stackoverflow.com/questions/8742804/ggplot2-add-a-gradient-colored-square-according-to-values/8743017#8743017

Here, the range of `y` is zero. In that case, the raster spans whole the plot region.
I had wanted to draw the horizontal bar using geom_raster, but it was impossible.
So I filed the issue w.r.t. the width/height of geom_raster in #350.
Does it make sense?