Why geom_raster() is not displaying the good columns orders in my heatmap?

I'm using a double triangular matrix (10x10) representing 2 types of correlations between these variables. The dataset is like this one (4x4).

ff <- data.frame(Var1=as.factor(c("V1", "V2", "V3","V4", "V1", "V2", "V3","V4", "V1", "V2", "V3","V4", "V1", "V2","V3","V4")),
                 Var2=as.factor(c("V1", "V1", "V1", "V1", "V2", "V2", "V2", "V2", "V3", "V3","V3","V3", "V4", "V4", "V4","V4")),
                 value=c(NA, 0.1, 0.2, 0.2, 
                         0.4, NA, 0.3, 0.4, 
                         0.5, 0.5, NA, 0.3,
                         0.2, 0.3, 0.3, NA))

I then used the geom_raster function from ggplot2 to get 2 colors scales (1 for each triangular matrix) + color for the identity diagonal in grey (ie correlation btw the same variable which does not exist (NA)). I used the same code than in a previous post (Single heatmap on two symetric matrices with different colours and scales R)

ggplot(ff, aes(Var1, Var2)) +
  # The first layer, with its own fill scale
  geom_raster(
    data = ~ subset(.x, as.numeric(Var1) > as.numeric(Var2)),
    aes(fill = value)
  ) +
  scale_fill_distiller(palette = "Blues") +
  # Declare new fill scale for the second layer
  new_scale_fill() +
  geom_raster(
    data = ~ subset(.x, as.numeric(Var1) < as.numeric(Var2)),
    aes(fill = value)
  ) +
  scale_fill_distiller(palette = "Reds", direction=-1) +
  # I'm not sure what to do with the diagonal. Make it grey?
  new_scale_fill() +
  geom_raster(
    data = ~ subset(.x, as.numeric(Var1) == as.numeric(Var2)),
    aes(fill = value)
  ) +
  scale_fill_distiller(palette = "Greys", guide = "none")

However, I'm a little disappointed because as you can see in the output I have (below) there is a problem in the final display of the graph I get :

As you can see what I expected to be the 1st column is now the last one and I don't know what to do. I tried to reorder the levels of the factor variable but it did not solve the problem. I would be very grateful if someone could help me for this challenge (for me !)

Many thanks in advance!

The problem comes from when you plot the first part of the heatmap (Var1 > Var2). In that case, Var1 (x-axis) starts at V2. And then when you plot the second, because the x-axis is discrete it goes to the right and V1 goes to the right.

First a hacky way would be to plot a grey (or whatever color you want) heatmap as a background and then plot the triangular heatmaps:

ggplot(ff, aes(Var1, Var2)) +
    geom_raster(fill = "darkgrey") +
    # The first layer, with its own fill scale
    geom_raster(
        data = ~ subset(.x, as.numeric(Var1) > as.numeric(Var2)),
        aes(fill = value)
    ) +
    scale_fill_distiller(palette = "Blues") +
    # Declare new fill scale for the second layer
    new_scale_fill() +
    geom_raster(
        data = ~ subset(.x, as.numeric(Var1) < as.numeric(Var2)),
        aes(fill = value)
    ) +
    scale_fill_distiller(palette = "Reds", direction=-1)

The second way would be to reorder your x-axis with scale_x_discrete:

ggplot(ff, aes(Var1, Var2)) +
    # The first layer, with its own fill scale
    geom_raster(
        data = ~ subset(.x, as.numeric(Var1) > as.numeric(Var2)),
        aes(fill = value)
    ) +
    scale_fill_distiller(palette = "Blues") +
    # Declare new fill scale for the second layer
    new_scale_fill() +
    geom_raster(
        data = ~ subset(.x, as.numeric(Var1) < as.numeric(Var2)),
        aes(fill = value)
    ) +
    scale_fill_distiller(palette = "Reds", direction=-1) +
    scale_x_discrete(limits = c("V1", "V2", "V3", "V4"))

Sorry I lost the diagonal part, it didn't work on my computer, but with first example you can customize the color of the background that will end on the diagonal.

I hope this answers your problem.