Gradient colour fill a barplot in ggplot2

Question

I'd like to gradient fill a ggplot2 bar plot based on the values of Y, but I'd like to manually set the colours. This example is almost what I need, but it sets the limits of the colour palette based on the values of Y, whereas I want to set these manually based on a biological stress threshold

Here's some sample code based on the linked example.

 d <- data.frame(x = 1:4, y= c (0.1, 0.3, 1, 0.6))
    
    # interpolate values from zero to y and create corresponding number of x values
    vals <- lapply(d$y, function(y) seq(0, y, by = 0.01))
    y <- unlist(vals)
    mid <- rep(d$x, lengths(vals))
    d2 <- data.frame(x = mid - 0.4,
                     xend = mid + 0.4,
                     y = y,
                     yend = y)
    
    ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) +
      geom_segment(size = 2) +
      scale_color_gradient2(low = "green4", mid = "yellow", high = "red", 
                            midpoint = max(d2$y)/2) +
      coord_cartesian(ylim = c(0 , 2))

I'd like the red to appear when the Y value is equal to 2, rather than the maximum value of Y in the data frame.

phalteman · Accepted Answer · 2022-01-06T06:02:56.530

If you know a priori what the values are that you'd like to be red (in your case, 2), you can just set the midpoint at half of that (1).

Your example, but with an extra data point to illustrate:

# d <- data.frame(x = 1:4, y= c(0.1, 0.3, 1, 0.6))
d <- data.frame(x = 1:5, y= c(0.1, 0.3, 1, 0.6, 2.0))
vals <- lapply(d$y, function(y) seq(0, y, by = 0.01))
y <- unlist(vals)
mid <- rep(d$x, lengths(vals))
d2 <- data.frame(x = mid - 0.4, xend = mid + 0.4, y = y, yend = y)

ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) +
  geom_segment(size = 2) +
  scale_color_gradient2(low = "green4", 
                        mid = "yellow", 
                        high = "red", 
                        midpoint = 1) + # changed here
  coord_cartesian(ylim = c(0 , 2))

As noted in the comments on the other answer, you won't see red in your original data - rerun the code here with your original dataset to see the result.

score 0 · Answer 2 · answered Jan 06 '22 at 05:46

0

Does this work? Just switched to scale_color_gradientn which allows us to use the values = call where we specify three values for the respective colours. (And a couple of other small changes.)

    
    ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) +
      geom_segment(size = 2) +
      scale_y_continuous(limits = c(0, 2)) +
      scale_color_gradientn(colours = c("green4", "yellow", "red"), 
                            values = c("0", max(d2$y)/2, "2"))

answered Jan 06 '22 at 05:46

Karthik Thrikkadeeri

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The colour guide produced won't have red though, since it will only show 0 to 1 (as in the data). – Karthik Thrikkadeeri Jan 06 '22 at 05:47
It works with the sample data, but try replacing the '1' with '2' in the original dataframe and it maxes out with yellow. Replacing the `midpoint` in the original code with '1', instead of `max(d2$y)/2` seems to do the trick `ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) + geom_segment(size = 2) + scale_color_gradient2(low = "green4", mid = "yellow", high = "red", midpoint = 1) + coord_cartesian(ylim = c(0 , 2))` – A.Benson Jan 06 '22 at 05:56
@A.Benson Which "1" are you replacing? But yes, as mentioned in the other answer, it makes sense to manually specify the midpoint if manually specifying the threshold of 2. – Karthik Thrikkadeeri Jan 06 '22 at 06:06

Gradient colour fill a barplot in ggplot2

2 Answers2