[R] Help with multicolored points in one plot

Deepayan Sarkar deepayan at stat.wisc.edu
Thu Feb 26 17:55:42 CET 2004

On Thursday 26 February 2004 10:13, David Thibault wrote:
> Hello all,
> I have a situation where I'd like to plot points from multiple groups of
> data in one plot.  I'd like each group's points to be colored a
> different color.  I've seen people comment on how you can alternate
> colors by providing a range of colors and then it will loop through each
> color as it plots individual points.  However, that just goes by
> individual points and not by group.
> Next I thought if, for example, I had 2 groups, I could make a 2 color
> vector.  Then I could merge the data from my two groups alternating into
> an X and Y vector, but that doesn't work either because the 2 groups may
> not have equal numbers of members.  For example
> -------------------------------
> Group1_Xdata <- c(1,2,3,4,5)
> Group2_Xdata <- c(10,20,30,40,50)
> Colors <- c("red","blue")
> Merged_XData <- c(1,10,2,10,3,30,4,40,5,50)
> -----------------------------
> That would work to make group1 red and group2 blue, but if Group 2 had 6
> members instead of 5, then the 6th member would come out red.
> Any thoughts?  Also, I want to be able to code this generically enough
> that I can have any number of groups and colors.

Trellis graphics (using the lattice package in R) has a fairly systematic 
approach for doing this. See example(xyplot) for some examples. For your 
data, a possible usage could be:

x <- c(Group1_Xdata, Group2_Xdata)
y <- c(Group1_Ydata, Group2_Ydata)
g <- c(rep(1, length(Group1_Xdata)), rep(2, length(Group2_Xdata)))

xyplot(y ~ x, groups = g)

or if you want to control the colors

xyplot(y ~ x, groups = g, col = c("red", "blue"))

With base graphics, something similar could be achieved with 

plot(x, y, col = c("red", "blue")[g])

lattice is more systematic in the sense that it has globally modifiable 
settings to control various graphical parameters (not only color) used to 
distinguish between groups, with reasonable defaults.



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