Using more than one `scale_fill_` in` ggplot2`

Question

I am trying to display multiple layers in ggplot2 , but I would like to use different scale_fill_ color schemes for each layer. I cannot do this, since calling something like scale_fill_gradientn twice just overwrites the first call with the second.

 library( ggplot2 ) library( reshape2 ) library( RColorBrewer ) set.seed( 123 ) 

I will first build the tile grid (note that I set the colors using scale_fill_gradientn ):

 foo <- matrix( data = rnorm( 100 ), ncol = 10 ) foo <- melt( foo ) plot <- ggplot() + geom_tile( data = foo, mapping = aes( x = Var1, y = Var2, fill = value ) ) + scale_fill_gradientn( colours = rev( brewer.pal( 7, "BrBG" ) ) ) plot 

Now I would like to add another plot on top of this, but with a unique color scheme. I can create a great storyline simply:

 bar <- data.frame( x = rnorm( 100, 4, 1 ), y = rnorm( 100, 6, 1.5 ) ) ggplot() + stat_density_2d( data = bar, mapping = aes( x = x, y = y, fill = ..level.. ), geom = "polygon" ) + scale_fill_gradientn( colours = rev( brewer.pal( 7, "Spectral" ) ) ) + xlim( 0, 10 ) + ylim( 0, 10 ) 

What I would like to do is to speak the second plot on top of the first, but keep the color schemes that you see above. If I try to just add a second layer on top of the first, I overwrite the original scale_fill_gradientn and force the two layers to share the same color scheme (which in this case also “compresses” the second layer, completely falling within the same color:

 plot <- plot + stat_density_2d( data = bar, mapping = aes( x = x, y = y, fill = ..level.. ), geom = "polygon" ) + scale_fill_gradientn( colours = rev( brewer.pal( 7, "Spectral" ) ) ) + xlim( 0, 10 ) + ylim( 0, 10 ) plot 

Is there a way to specify separate color schemes for each layer? I noticed that, for example, stat_density_2d understands the aesthetics of colour , but I tried to specify it to no avail (it only adds color as a label in the legend and returns the default color scheme):

 ggplot() + stat_density_2d( data = bar, mapping = aes( x = x, y = y, fill = ..level.., colour = "red" ), geom = "polygon" ) + xlim( 0, 10 ) + ylim( 0, 10 ) 

I feel that there should be a different way to set the color scheme to “per layer”, but I was clearly looking for the wrong places.

Answer 1

One way to get around the restriction is to match the color instead (as you have already hinted). Here's how to do it:

We save the base raster chart, and then add:

 plot + stat_density_2d( data = bar, mapping = aes( x = x, y = y, col = ..level.. ), geom = "path", size = 2 ) + scale_color_gradientn( colours = rev( brewer.pal( 7, "Spectral" ) ) ) + xlim( 0, 10 ) + ylim( 0, 10 ) 

This gives us:

This is not entirely satisfactory, mainly because the scales have a lot of perceptual overlap (I think). A game with different scales can give us the best result:

 plot <- ggplot() + geom_tile( data = foo, mapping = aes( x = Var1, y = Var2, fill = value ) ) + viridis::scale_fill_viridis(option = 'A', end = 0.9) plot + stat_density_2d( data = bar, mapping = aes( x = x, y = y, col = ..level.. ), geom = "path", size = 2 ) + viridis::scale_color_viridis(option = 'D', begin = 0.3) + xlim( 0, 10 ) + ylim( 0, 10 ) 

In my opinion, this is not very good (using multiple color scales is confusing), but much more bearable.