[R] Question about graphics in R
Duncan Murdoch
murdoch at stats.uwo.ca
Sun Jan 8 04:06:22 CET 2006
On 1/7/2006 12:47 PM, Martin Erwig wrote:
> Considering the R function/plot shown below, I wonder whether
> it is possible to do the following changes:
>
> (1) Change the color of each point to be picked from
> list of colors according to its z-value. (The range
> should be from blue (z=0) to red (z=1).) The grid
> should then be omitted. [I have seen "terrain.colors", but
> don't know how to use it for this purpose.]
>
> (2) Add two lines to the surface for, say z=0.8 and
> z=0.3. [Can contour or contourLines be used?]
>
>
> ---
>
> x <- seq(0, 1, length = 50)
> y <- x
> f <- function(x,y) { sin ((1-x)*y) }
> z <- outer(x,y,f)
>
> persp(x, y, z,
> theta = 30, phi = 30,
> shade = 0.3, col = "red"
> )
It's not that hard, but there are a couple of tricks to it. The
colorRamp() and colorRampPalette() functions give you the way to go from
z values to colors. persp() wants output in the form colorRampPalette
gives, so use that.
But the col option to persp() wants one color per facet, not one per z
value, so you need to be a little careful calculating them. However,
this should give what you want:
colfn <- colorRampPalette(c("blue","white","red"))
# You don't need white in the middle, but I think it looks better
cols <- colfn(256)[1 + 255*z[1:49, 1:49]]
trans <- persp(x, y, z, theta=30, phi=30, col = cols)
To add the lines, use contourLines() to calculate them, then trans3d()
(from the ?persp examples) to convert them to coordinates that can be
plotted. E.g.
clines <- contourLines(x,y,z, levels=c(0.8, 0.3))
trans3d <- function(x,y,z, pmat) {
tr <- cbind(x,y,z,1) %*% pmat
list(x = tr[,1]/tr[,4], y= tr[,2]/tr[,4])
}
for (i in seq(along=clines)) lines(trans3d(clines[[i]]$x, clines[[i]]$y,
clines[[i]]$level, trans))
> ---
>
>
> Finally, I would also produce a flattened 2D map
> of the same function, i.e. a map in which each point
> (x,y) is mapped to a color in a range according to
> f(x,y). Also two lines for f(x,y)=c1 and f(x,y)=c2
> should be added.
Use image() for that, e.g.
image(x,y,z)
and then contour():
contour(x,y,z,levels=c(0.8,0.3),add=T)
Duncan Murdoch
>
> Is this possible?
>
> I would be grateful for any hints.
>
>
> Thanks,
> Martin
>
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