How to customize the shape of an contour made inside a geom_tile in ggplot2?

Question

An X, Y, and Z dataset was plotted using geom_tile. I want to make a contour line around the tiles with values less than or equal to 2. For this, I used the stat_contour function, but the result was not as expected. How can I get the expected result? (last image)

library(ggplot2)

X <- 1:3
Y <- seq(0,20,10)

df <- expand.grid(X = X, Y = Y)

df$Z <- c(5,4,9,2.1,1.5,1.2,6,7,1.9)  

ggplot(df, aes(X, Y)) +
  geom_tile(aes(fill = Z)) +
  scale_fill_distiller(palette = "RdYlGn") +
  stat_contour(aes(z = Z),
               breaks = 2,
               color = 1)

I want something like:

Answer 1

You could try adding a different layer with a subset of the data containing only the rows where Z <= 2. The last layer is so that there are no black lines within the block of tiles (there might be an argument in geom_tile that I am not aware of that does this).

layer <- df %>% filter(Z <= 2)

ggplot(df, aes(X, Y)) +
  geom_tile(aes(fill = Z)) +
  scale_fill_distiller(palette = "RdYlGn") +
geom_tile(data=layer, alpha = 0.0, color = "black", size = 1, linejoin = "round") +
  geom_tile(data=layer, alpha = 1, aes(fill = Z))