CODEWITHSUNDEEP
rggplot2alpha-transparency
Sam Firke
Sam Firke

Reputation: 23014

How to adjust relative transparency of ggplot2 points

I want to make some points stand out on a ggplot2 chart by giving them less transparency while the rest fade to the background. But no matter what two alpha values I give the sets of points, their relative transparency is the same.

Here's 0.8 vs 0.7:

x <- mtcars
x$opacity <- ifelse(x$cyl == 6, 0.8, 0.7)

ggplot(x, aes(x = wt, y = mpg, color = cyl, alpha = opacity)) +
  geom_point()

ggplot2 scatterplot with 0.8 and 0.7 alpha for two sets of points

And here's 0.8 vs 0.1 -- looks the same:

x$opacity <- ifelse(x$cyl == 6, 0.8, 0.1)

ggplot(x, aes(x = wt, y = mpg, color = cyl, alpha = opacity)) +
  geom_point()

ggplot2 scatterplot with 0.8 and 0.1 alpha for two sets of points How can I fine-tune that relative alpha so that the two sets are closer in transparency? Right now the values of the two numbers don't seem to matter. Specifically, in this case I want the darker points (with the higher alpha) to be more transparent.

Upvotes: 2

Views: 3796

Answers (3)

alistaire
alistaire

Reputation: 43334

You're mapping the values 0.7 and 0.8 to alpha, not necessarily using them for alpha. A quicker way is to map the condition and then set alpha:

library(ggplot2)

ggplot(mtcars, aes(x = wt, y = mpg, color = cyl, alpha = cyl == 6)) +
    geom_point() + 
    scale_alpha_discrete(range = c(0.2, 0.8))
#> Warning: Using alpha for a discrete variable is not advised.

points with alpha

Upvotes: 1

MrFlick
MrFlick

Reputation: 206207

Since you are trying to pass actual alpha values to the aesthetic mapping, be sure to use

scale_alpha_identity()

Otherwise ggplot will rescale your values just like it created the colors for you automatically.

Upvotes: 4

CFD
CFD

Reputation: 198

Add scale_alpha_continuous to your plot and define the range. e.g.

scale_alpha_continuous(range = c(0.7, 0.8))

Upvotes: 1

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