Create heatmap with values from matrix in ggplot2

Key is to add a row identifier to the data and shape it “longer”.

edit Dec 2022 to make code reproducible with R 4.2.2 / ggplot2 3.4.0 and reflect changes in tidyverse semantics

library(ggplot2)
library(tidyverse)
dat <- matrix(rnorm(100, 3, 1), ncol = 10)
## the matrix needs names
names(dat) <- paste("X", 1:10)

## convert to tibble, add row identifier, and shape "long"
dat2 <-
  dat %>%
  as_tibble() %>%
  rownames_to_column("Var1") %>%
  pivot_longer(-Var1, names_to = "Var2", values_to = "value") %>%
  mutate(
    Var1 = factor(Var1, levels = 1:10),
    Var2 = factor(gsub("V", "", Var2), levels = 1:10)
  )
#> Warning: The `x` argument of `as_tibble.matrix()` must have unique column names if
#> `.name_repair` is omitted as of tibble 2.0.0.
#> ℹ Using compatibility `.name_repair`.

ggplot(dat2, aes(Var1, Var2)) +
  geom_tile(aes(fill = value)) +
  geom_text(aes(label = round(value, 1))) +
  scale_fill_gradient(low = "white", high = "red")

^{Created on 2022-12-31 with reprex v2.0.2}