trisurf data 3dplot with NaN values

I'm trying to plot some 3d data. Some values are not valid and should be ignored by setting them to NaN (let's say all x<y). My Code looks like..

[x,y] = meshgrid(1:3,1:5);
tri = delaunay(x,y);
trisurf(tri,x,y,data);

The attached graphic shows my problem. There is data at (1,1) and I need a triangle between the points (1,1),(2,1),(2,2) and (2,2),(3,2),(3,3). The solution seems to be simple but I'm trying to solve this for quite a while now. Thanks for help!