how to draw the legend about color from 3d picture

I have drawed a picture with persp, it's 3d map with different color,
indicate different altitude. In gnuplot, the corresponding command
'splot' will generate a picture beside to indicate the relationship
between color and altitude. But in R, how to draw it? I have read the
manual of legend, but they are all about how to draw a legend with
colored text, not a continuous varing color with corresponding number.

I don't think there's an automatic way to do this (though probably some 
package provides one).

If you want to write your own, take a look at example(filled.contour). 
The legend it draws is done by this code:

     plot.new()
     plot.window(xlim = c(0, 1), ylim = range(levels), xaxs = "i",
         yaxs = "i")
     rect(0, levels[-length(levels)], 1, levels[-1], col = col)
     if (missing(key.axes)) {
         if (axes)
             axis(4)
     }
     else key.axes
     box()

(but it has done a lot of setup using layout() and par() before this).

Duncan Murdoch