Legend for confusion matrix in R

Question

I have created a confusion matrix plot in R however I would like to get the legend that shows the values for the colours. How would I do this? So far I have the following:

z = c(135,9,0,7,0,0,55,3,
      3,260,0,1,0,0,104,2,
      0,0,0,21,0,0,56,16,
      1,1,0,155,0,0,66,45,
      0,0,0,134,0,0,28,11,
      0,0,0,10,0,0,4,24,
      7,7,0,31,0,0,454,6,
      0,0,0,73,0,0,21,181)

z = matrix(z, ncol=8)
colnames(z) = c("Water","Shrubs","Commercial Agriculture","Residential Areas","Subsistence Farming","Roads","Grassland","Degraded Areas")

rownames(z) = c("Water","Shrubs","Commercial Agriculture","Residential Areas","Subsistence Farming","Roads","Grassland","Degraded Areas")

image(z[,ncol(z):1], axes=FALSE)

axis(2, at = seq(0, 1, length=length(colnames(z))), labels=colnames(z))

heatmap(t(z)[ncol(z):1,], Rowv=NA,
         Colv=NA, col = heat.colors(256))

Answer 1

I suggest using tidyverse and ggplot. Sample code below. With the values parameter in scale_fill_gradientn you can set the values for the individual colors. For other options, I recommend ?scale_fill_gradientn - there are a lot of them.

library(tidyverse)
library(RColorBrewer) # only for color set

z = c(135,9,0,7,0,0,55,3,
      3,260,0,1,0,0,104,2,
      0,0,0,21,0,0,56,16,
      1,1,0,155,0,0,66,45,
      0,0,0,134,0,0,28,11,
      0,0,0,10,0,0,4,24,
      7,7,0,31,0,0,454,6,
      0,0,0,73,0,0,21,181)

z = matrix(z, ncol=8)
colnames(z) = c("Water","Shrubs","Commercial Agriculture","Residential Areas","Subsistence Farming","Roads","Grassland","Degraded Areas")

rownames(z) = c("Water","Shrubs","Commercial Agriculture","Residential Areas","Subsistence Farming","Roads","Grassland","Degraded Areas")

z2 <- reshape2::melt(z) %>% # converting data to tibble
  as_tibble()

ggplot(z2, aes(x = Var1, y = Var2, fill = value))+
  geom_raster()+
  scale_fill_gradientn(colours = rev(brewer.pal(9, "YlOrRd")))+ # you can use a different palette, I use RColorBrewer, but it can be e.g. scale_fill_continous
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 0.5))

^{Created on 2022-09-30 with reprex v2.0.2}