How to plot heatmap with multiple categories in a single cell with ggplot2? Heatmap plot of categorical variables could be done with this code

#data 
datf <- data.frame(indv=factor(paste("ID", 1:20),
    levels =rev(paste("ID", 1:20))), matrix(sample(LETTERS[1:7], 400, T), ncol = 20))



library(ggplot2); 
library(reshape2)
# converting data to long form for ggplot2 use

datf1 <- melt(datf, id.var = 'indv')

ggplot(datf1, aes(variable, indv)) + geom_tile(aes(fill = value),
   colour = "white")  +   scale_fill_manual(values= rainbow (7))

The codes came from here: http://rgraphgallery.blogspot.com/2013/04/rg54-heatmap-plot-of-categorical.html

But what about multiple categories in a single cell like this? Is it possible to use triangle or other shape as a cell?

http://postimg.org/image/4dudrv0nz/