On 5/21/09, Barry Rowlingson <b.rowling...@lancaster.ac.uk> wrote: > On Thu, May 21, 2009 at 2:18 PM, <richard.cot...@hsl.gov.uk> wrote: > > > Most of the plots where colour is typically used to signify a variable > > already do map colours to data values. Take a look at help pages for > > levelplot/contourplot/wireframe from the lattice package, and image from > > base graphics. > > > > (The format is typically slightly different to your suggested > > specification, though the principle is the same. The functions take a > > vector of cut points, and a vector of colours.) > > > The problem here is that the user doesn't have exact control of the > mapping from value to colour. For example (using a slightly more > safe-for-use-after-lunch version of the levelplot example grid): > > x <- seq(pi/4, 5 * pi, length.out = 100) > y <- seq(pi/4, 5 * pi, length.out = 100) > r <- as.vector(sqrt(outer(x^2, y^2, "+"))) > grid <- expand.grid(x=x, y=y) > grid$z <- r > grid$z2 = r *0.5 > > > Then I do: > > levelplot(z~x*y, grid, cuts = 5, col.regions=rainbow(5)) > > very nice, but suppose I want to show $r2 on the same colour scale, I > can't just do: > > levelplot(z2~x*y, grid, cuts = 5, col.regions=rainbow(5)) > > because that looks the same as the first one since levelplot uses the > whole colour range.
But you could specify an explicit 'at' vector specifying the color breakpoints: effectively, you want at = do.breaks(zlim, 5). lattice does have a function called 'level.colors' that factors out the color assignment computation. -Deepayan > > > The base graphics "image" function has zlim arguments which let you do: > > z=outer(1:10,1:10,"*") > image(z) > image(z/2, zlim=range(z)) > > but again, not obvious, and complex/impossible when using more > sophisticated colour mappings. > > > > There may be some utility in creating functions to generate these colour > > maps outside of the plotting functions, if only so that the code can be > > recycled for new functions. > > > Exactly, it would make a new package. > > > Barry > > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
