add explanation

src/backends/gr.jl

@@ -1537,6 +1537,7 @@ function gr_add_series(sp, series)
         dmin, dmax = GR.gr3.volume(y.v, 0)
     elseif st in (:heatmap, :image)
         if !ispolar(series)
+            # `z` is already transposed, so we need to reverse before passing its size.
             x, y = heatmap_edges(x, xscale, y, yscale, reverse(size(z)))
         end
         if st === :heatmap