Relative color scale in ggplot

[Michael]

Hi all,

I have a ggplot where colors scale by default to the global min and max of a variable. I'd like to scale the color to the max response within a particular x value. An example may be clearer:

set.seed(1)

df <- data.frame(x=c(rep("a", 10), rep("b", 10)),

                 y=c(sample(1:4, 10, replace=T), sample(1:6, 10, replace=T)))

ggplot(data=df, aes(x=x, y=y)) + 

  stat_sum(aes(group=x, color=y)) +

  scale_size_area(max_size=15) +

  scale_color_gradient(low="red", high="green")

That produces the following.

What I'd like is for the point at (a, 4) to be the same color as the one at (b, 6). There's probably a simple solution, but I'm stuck. Thanks in advance,

Michael

[Noam Ross]

You can normalize the y values within the x groups and use these to assign colors. The numbers in the legend become meaningless, though, and you’ll want to modify or get rid of it:

require(ggplot2)
require(plyr)
set.seed(1)
df <- data.frame(x=c(rep("a", 10), rep("b", 10)),
                 y=c(sample(1:4, 10, replace=T), sample(1:6, 10, replace=T)))
df <- ddply(df, .(x), transform, y2=(y-min(y))/(max(y)-min(y)))  # <--- Normalized y values are "y2"
ggplot(data=df, aes(x=x, y=y)) + 
  stat_sum(aes(group=x, color=y2)) +        # <---  Assign color to "y2"
  scale_size_area(max_size=15) +
  scale_color_gradient(low="red", high="green", breaks=c(0,1),  # <--- Modify the scale labels
                       label=c("min.group", "max.group"))

You get:

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[Michael Levy]

Perfect. Thanks, Noam!

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Michael Levy
c: 304-376-4523