How to add text annotation to heatmap with ggplot2

In this post, we will see how can we add text annotation to heatmaps made with ggplot2. In ggplot2, we can make simple heatmaps using ggplot2’s geom_raster() and geom_tile(). In this post, we will use geom_text() to add text annotation, i.e. numerical values in the heatmap. We will also see multiple examples of adding color annotation based the numerical values visualized in the heatmap.

library(tidyverse)
library(tidyquant)
theme_set(theme_bw(16))
library(glue)

Let us make a heatmap of monthly returns of Nvidia stock since 1999.

start_date <- as.Date("1980-01-01")
end_date <- Sys.Date()
company_name <- "Nvidia"
stock_ticker <- "NVDA"
print(stock_ticker)
## [1] "NVDA"

We use tidyquant package to get the stock data

stock_df <- tq_get(stock_ticker,
                   from = start_date,
                   to = end_date)

stock_df |> head()

## # A tibble: 6 × 8
##   symbol date         open   high    low  close     volume adjusted
##   <chr>  <date>      <dbl>  <dbl>  <dbl>  <dbl>      <dbl>    <dbl>
## 1 NVDA   1999-01-22 0.0437 0.0488 0.0388 0.0410 2714688000   0.0376
## 2 NVDA   1999-01-25 0.0443 0.0458 0.0410 0.0453  510480000   0.0416
## 3 NVDA   1999-01-26 0.0458 0.0467 0.0411 0.0418  343200000   0.0383
## 4 NVDA   1999-01-27 0.0419 0.0430 0.0396 0.0417  244368000   0.0382
## 5 NVDA   1999-01-28 0.0417 0.0419 0.0413 0.0415  227520000   0.0381
## 6 NVDA   1999-01-29 0.0415 0.0417 0.0396 0.0396  244032000   0.0363

And compute monthly returns of the stock since 1999

monthly_return <- stock_df %>%
    tq_transmute( select     = adjusted,
               mutate_fun = periodReturn,
               period     = "monthly") %>%
    mutate(return= ifelse(monthly.returns > 0, "+ve", "-ve")) %>%
    mutate(year=factor(year(date)) ) %>%
    mutate(month=month(date, label=TRUE))

We have the data ready for making a heatmap.

monthly_return |>
  head()
## # A tibble: 6 × 5
##   date       monthly.returns return year  month
##   <date>               <dbl> <chr>  <fct> <ord>
## 1 1999-01-29         -0.0349 -ve    1999  Jan  
## 2 1999-02-26          0.155  +ve    1999  Feb  
## 3 1999-03-31         -0.0370 -ve    1999  Mar  
## 4 1999-04-30         -0.136  -ve    1999  Apr  
## 5 1999-05-28         -0.0651 -ve    1999  May  
## 6 1999-06-30          0.121  +ve    1999  Jun

Heatmap with ggplot2’s geom_raster()

We can make a heat map with ggplot2’s geom_raster() function. Here is a heatmap of monthly returns of Nvidia over the years using geom_raster()

monthly_return |> 
  ggplot(aes(x=month, y=year, fill=monthly.returns))+
  geom_raster()
ggsave("heatmap_with_geom_raster_ggplot2.png")

Heatmap with ggplot2’s

We can also use geom_tile() fin gggplot2 and make a heatmap. Here is the same heatmap of monthly returns of Nvidia over the years but using geom_tile()

monthly_return |> 
  ggplot(aes(x=month, y=year, fill=monthly.returns))+
  geom_tile()
ggsave("heatmap_with_geom_tile_ggplot2.png")

monthly_return |> 
  ggplot(aes(x=month, y=year, fill=return))+
  geom_raster()+
  geom_text(aes(label = paste0(round(monthly.returns*100,1), "%")))+
  scale_fill_brewer(palette = "Dark2")
ggsave("heatmap_with_text_color_annotation_ggplot2.png")

monthly_return |> 
  ggplot(aes(x=month, y=year, fill=monthly.returns))+
  geom_raster()+
  geom_text(aes(label = paste0(round(monthly.returns*100,1), "%")))+
  scale_fill_gradient2(midpoint = 0, mid="white", low="#dc322f", high="#008000")
ggsave("text_annotation_to_heatmap_ggplot2.png", width=10, height=11)

monthly_return |> 
  ggplot(aes(x=month, y=fct_rev(year), fill=monthly.returns))+
  geom_raster()+
  geom_text(aes(label = paste0(round(monthly.returns*100,1), "%")),fontface = "bold")+
  scale_fill_gradient2(midpoint = 0, mid="white", low="#dc322f", high="#008000")+
  labs(title=glue("{company_name} ({stock_ticker}): Monthly Returns Heatmap"),
       x=NULL, y=NULL)+
  theme(legend.position = "none")
ggsave("customizing_text_annotation_to_heatmap_ggplot2.png", width=10, height=11)