Q:

How to create an x=y-axis

I have this wave in Maple, but I don't want to have the whole square showing, I just want one of the triangular parts visible. Hope that makes sense. Basically I need to axis to go from say x=0..1, y=0..1 and x=y. It is this x=y axis which I can't get.
This is the code which I have so far.
with(plots):
animate(plot3d,[sin(Pi*2*x)*sin(Pi*y)*sin(t)-sin(Pi*x)*sin(Pi*2*y)*sin(t),x=0..1, y=0..1],t=0..2*Pi);

A:

The plot3d command offers a syntax where either (or both) of the endpoints in the range supplied for one of the plotting variables can be an expression in terms of the other plotting variable.
So instead of x=0..1, y=0..1 to denote a rectangular domain you could use x=0..y, y=0..1 to denote a triangular domain. The remaining triangular domain could be denoted using x=y..1, y=0..1.
You utilized this functionality in another of your recent questions.
with(plots):

animate(plot3d,[sin(Pi*2*x)*sin(Pi*y)*sin(t)
                -sin(Pi*x)*sin(Pi*2*y)*sin(t),
                x=0..y, y=0..1],
        t=0..2*Pi);

And the remaining portion could be had by,
animate(plot3d,[sin(Pi*2*x)*sin(Pi*y)*sin(t)
                -sin(Pi*x)*sin(Pi*2*y)*sin(t),
                x=y..1, y=0..1],
        t=0..2*Pi);

