SHADOWPLOT Add a shadow to an existing surface plot

For some surface plots, it can be helpful to visualize the shadow (2D projection) of the surface. This can give a quick perspective on the data's variance.

SHADOWPLOT PLANE Adds a shadow plot on the PLANE boundary
PLANE can be:
x, y, or z: Plots on back/top wall of x, y or z
1 .. 6 : Plots on Nth wall, numbered as in AXIS:
[xmin xmax ymin ymax zmin zmax]

SHADOWPLOT(HAX,PLANE) Adds a shadow plot on the Nth wall on axes HAX
HS = SHADOWPLOT(...) Returns a handle to the shadow (a patch)

Examples:
figure
surf(peaks)
shading interp
shadowplot x % Back X Wall
shadowplot y % Back Y Wall