Plotting a line with a certain condition in geom_tile

Question

I'm plotting my data in a heatmap. Some of the data of the fill is positive and some is negative. I would like to plot a line over the heatmap that separates what is positive and what is negative.

I have looked up and tried using geom_contour, but couldn't figure out if there was an argument where to introduce the condition fill>0 or something like that. this is my current plot.

I would like a line that separates positive fron negative tiles like this

but keeping the color gradient of the oiginal, just plotting a line that separates both spaces.

Answer 1

You can compute in a separate dataframe the threshold of y above which the fill value is positive and add it to your plot with geom_step():

Reproducible example

library(dplyr)
library(ggplot2)

df <- data.frame(x = rep(1:5, 5),
                 y = rep(1:5, each = 5),
                 z = -12:12) 

ggplot(df, aes(x = x, y = y, fill = z)) +
  geom_tile() + scale_fill_viridis_c() + theme_minimal()

Compute the steps

df2 <- df %>% 
  filter(z >= 0) %>% 
  group_by(x) %>% 
  filter(y == min(y)) %>%
  mutate(x = x - .5, y = y - .5) %>%
  arrange(x)

df2 <- df2 %>% bind_rows(df2[nrow(df2), ] %>% mutate(x = x + 1))
df2

# A tibble: 6 x 3
# Groups:   x [6]
      x     y     z
  <dbl> <dbl> <int>
1   0.5   3.5     3
2   1.5   3.5     4
3   2.5   2.5     0
4   3.5   2.5     1
5   4.5   2.5     2
6   5.5   2.5     2

Plot

ggplot() +
  geom_tile(data = df, aes(x = x, y = y, fill = z)) +
  geom_step(data = df2, aes(x = x, y = y), size = 2) +
  scale_fill_viridis_c() + theme_minimal()