I don't have your data, so I cannot use it. But it is simple enough to generate some simple non-uniform data.
xyz = sin(rand(10000,3)*2);
n = size(xyz,1);
H1 = plot3(xyz(:,1),xyz(:,2),xyz(:,3),'b.');
set(H1,'Markersize',.1)
hold on
box on
grid on
H2 = plot3(xyz(:,1),xyz(:,2),repmat(min(xyz(:,3)),n,1),'r.');
set(H2,'Markersize',.1)
The red dots all lie in the projection plane.
If you have vastly more data than this, then you might use smaller dots. But I'll admit that MATLAB graphics seem to lag a bit when trying to plot millions of points. With millions of points, you might also want to use tools like accumarray, to actually count the number of points that lie in any cell of the projection plane.
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