Creating a vertical color gradient for a geom_bar plot

Question

I have searched and searched, but I cant seem to find an elegant way of doing this!

I have a dataset Data consisting of Data$x (dates) and Data$y (numbers from 0 to 1)

I want to plot them in a bar-chart:

ggplot(Data) + geom_bar(aes(x = x, y = y, fill = y, stat = "identity")) +
   scale_fill_gradient2(low = "red", high = "green", mid = "yellow", midpoint = 0.90)

The result looks like this

However, I wanted to give each bar a gradient in the vertical direction ranging from 0 (red) to y (greener depending on y). Is there any way of doing this smoothly?

I have tried to see if I could impose a picture on the graph as a hack, but I can't impose it on the bars only except in a super super ugly way.

Answer 1

Doesn't exist as far as I know, but you can manipulate your data to produce it.

library(ggplot2)

df = data.frame(x=c(1:10),y=runif(10))

prepGradient <- function(x,y,spacing=max(y)/100){
  stopifnot(length(x)==length(y))
  df <- data.frame(x=x,y=y)
  newDf = data.frame(x=NULL,y=NULL,z=NULL)
  for (r in 1:nrow(df)){
    n <- floor(df[r,"y"]/spacing)
    for (s in c(1:n)){
      tmp <- data.frame(x=df[r,"x"],y=spacing,z=s*spacing)
      newDf <- rbind(newDf,tmp)
    }
    tmp <- data.frame(x=df[r,"x"],y=df[r,"y"]%%spacing,z=df[r,"y"])
    newDf <- rbind(newDf,tmp)
  }
  return(newDf)
}

df2 <- prepGradient(df$x,df$y)

ggplot(df2,aes(x=x,y=y,fill=z)) + 
  geom_bar(stat="identity") + 
  scale_fill_gradient2(low="red", high="green", mid="yellow",midpoint=median(df$y))+
  ggtitle('Vertical Gradient Example') +
  theme_minimal()

Answer 2

Found a less hacky way to do this when answering Change ggplot bar chart fill colors

library(tidyverse)

df <- data.frame(value = c(20, 50, 90),
                 group = c(1, 2, 3))

df_expanded <- df %>%
  rowwise() %>%
  summarise(group = group,
            value = list(0:value)) %>%
  unnest(cols = value)

df_expanded %>%
  ggplot() +
  geom_tile(aes(
    x = group,
    y = value,
    fill = value,
    width = 0.9
  )) +
  coord_flip() +
  scale_fill_viridis_c(option = "C") +
  theme(legend.position = "none")

Answer 3

Because this did not explicitly ask for divergent / multi-hue scales (in the title), here a simple hack for a single-hue gradient. This is very much the approach like suggested for a gradient fill under a curve as seen here

library(ggplot2)
d <- data.frame(x = 1:3, y = 1:3)
n_grad <- 1000
grad_df <- data.frame(yintercept = seq(0, 3, len = 200), 
                      alpha = seq(0.3, 0, len = 200))
ggplot(d ) +
geom_col(aes(x, y), fill = "darkblue") +
  geom_hline(data = grad_df, aes(yintercept = yintercept, alpha = alpha), 
             size = 1, colour = "white", show.legend = FALSE) + 
## white background looks nicer then 
theme_minimal()