ggplot2 heatmap with tile height and width as aes()

Question

I'm trying to create a heat map for an OD matrix, but I wanted to scale the rows and columns by certain weights. Since these weights are constant across each category I would expect the plot would keep the rows and columns structure.

# Tidy OD matrix
df <- data.frame (origin  = c(rep("A", 3), rep("B", 3),rep("C", 3)),
                  destination = rep(c("A","B","C"),3),
                  value = c(0, 1, 10, 5, 0, 11, 15, 6, 0))

# Weights
wdf <- data.frame(region = c("A","B","C"),
                  w = c(1,2,3))

# Add weights to the data.
plot_df <- df %>% 
  merge(wdf %>% rename(w_origin = w), by.x = 'origin', by.y = 'region') %>% 
  merge(wdf %>% rename(w_destination = w), by.x = 'destination', by.y = 'region')
  

Here's how the data looks like:

> plot_df
  destination origin value w_origin w_destination
1           A      A     0        1             1
2           A      C    15        3             1
3           A      B     5        2             1
4           B      A     1        1             2
5           B      B     0        2             2
6           B      C     6        3             2
7           C      B    11        2             3
8           C      A    10        1             3
9           C      C     0        3             3

However, when passing the weights as width and height in the aes() I get this:

ggplot(plot_df, 
       aes(x = destination, 
           y = origin)) +
  geom_tile(
    aes(
      width = w_destination,
      height = w_origin,
      fill = value),
    color = 'black')

It seems to be working for the size of the columns (width), but not quite because the proportions are not the right. And the rows are all over the place and not aligned.

I'm only using geom_tile because I could pass height and width as aesthetics, but I accept other suggestions.

Answer 1

The issue is that your tiles are overlapping. The reason is that while you could pass the width and the heights as aesthetics, geom_tile will not adjust the x and y positions of the tiles for you. As your are mapping a discrete variable on x and y your tiles are positioned on a equidistant grid. In your case the tiles are positioned at .5, 1.5 and 2.5. The tiles are then drawn on these positions with the specified width and height.

This could be easily seen by adding some transparency to your plot:

library(ggplot2)
library(dplyr)

ggplot(plot_df, 
       aes(x = destination, 
           y = origin)) +
  geom_tile(
    aes(
      width = w_destination,
      height = w_origin,
      fill = value), color = "black", alpha = .2)

To achieve your desired result you have to manually compute the x and y positions according to the desired widths and heights to prevent the overlapping of the boxes. To this end you could switch to a continuous scale and set the desired breaks and labels via scale_x/y_ continuous:

breaks <- wdf %>% 
  mutate(cumw = cumsum(w),
         pos = .5 * (cumw + lag(cumw, default = 0))) %>% 
  select(region, pos)

plot_df <- plot_df %>% 
  left_join(breaks, by = c("origin" = "region")) %>% 
  rename(y = pos) %>% 
  left_join(breaks, by = c("destination" = "region")) %>% 
  rename(x = pos)

ggplot(plot_df, 
       aes(x = x, 
           y = y)) +
  geom_tile(
    aes(
      width = w_destination,
      height = w_origin,
      fill = value), color = "black") +
  scale_x_continuous(breaks = breaks$pos, labels = breaks$region, expand = c(0, 0.1)) +
  scale_y_continuous(breaks = breaks$pos, labels = breaks$region, expand = c(0, 0.1))