Plotting magnitude of a vector

[prj]

So common that it writes literally as you want it to be written.

mesh Th = square(10, 10);
fespace Wh(Th, [P2, P2]);
fespace Vh(Th, P2);
Wh [ux, uy] = [x, y];
Vh magnitude = sqrt(ux^2+uy^2);
plot(magnitude);