[R] how to draw the legend about color from 3d picture

[yk]

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.