creating a more continuous color palette in r, ggplot2, lattice, or latticeExtra

Question

Warning.... very novice question follows:

I am trying to plot a fairly regular distribution of several thousand (X,Y) points each associated with a value, let\'s cal

Answer 1

Have you looked at scale_gradient in ggplot? Or scale_brewer for discrete colours? Here's an example of scale_gradient

dat <- data.frame(x = rnorm(1000), y = rnorm(1000), z = sample(-20:20, 1000, TRUE))

p <- ggplot(dat, aes(x, y, colour = z)) + geom_point() 
p + scale_colour_gradient()
p + scale_colour_gradient(low = "red", high = "blue")
p + scale_colour_gradient2(low = "red", mid = "white", high = "blue")

Answer 2

As far as lattice is concerned, you can set up your colors palette with RColorBrewer (or even colorspace). Using the example provided by @Chase, but with positive value for z:

dat <- data.frame(x = rnorm(1000), y = rnorm(1000), z = sample(0:40, 1000, TRUE))
library(RColorBrewer)
# see, e.g.
# display.brewer.all(9, type="seq")
# display.brewer.pal(11, "RdBu")
my.col <- colorRampPalette(brewer.pal(11, "RdBu"))(diff(range(dat$z)))
xyplot(y ~ x, data=dat, col=my.col[dat$z], pch=19, alpha=.5)

Note that it is also necessary here to increase the range of available colors by interpolation. Also, with levelplot(), you might want to play with cut= and pretty=.

Answer 3

The "concept" you are missing is the at argument to levelplot() which defines the breakpoints between colour levels and/or contour lines. The default is pretty(z) which results in only a few levels. You can set at to be a sequence covering the range of values you want.

library(latticeExtra)

dat <- data.frame(x = rnorm(1000), y = rnorm(1000), z = rnorm(1000, mean = 1))
## for centering the colour key around zero
maxz <- max(abs(dat$z))

levelplot(z ~ x * y, dat, at = seq(-maxz, maxz, length = 100), 
    panel = panel.levelplot.points, par.settings = custom.theme.2())