Merge pull request #1189 from daschw/spy

use fg_color for matrices with unique non-zeros in spy recipe

src/recipes.jl

@@ -937,8 +937,7 @@ end
     mat = g.args[1]
     if length(unique(mat[mat .!= 0])) < 2
         legend --> nothing
-        # revert the default gradient to have a darker (almost black) color in the default theme
-        seriescolor --> cgrad([cgrad()[0], cgrad()[1]])
+        seriescolor --> cgrad([invisible(), fg_color(plotattributes)])
     end
     n,m = size(mat)
     Plots.SliceIt, 1:m, 1:n, Surface(mat)