Why is visualization of stresses is not continuous?

I tried to perform a modal analysis with your toolbox but don’t obtain good results. The resulting modal form is similar to other solvers such as Ansys, but the plot of stresses is not continuous. Can you tell me me what could be wrong?

The default finite element shape functions used are 1:st order linear basis functions, and stresses are 1:st derivatives so when chosen will be piecewise constant on each element.

You can improve this by selecting "FEM Discretization" > "(P2/Q2) second order conforming" (or higher finite element shape/basis functions) in the Equation Settings dialog box (resulting in a linear representation of the stresses on each cell).