If you plot a dependent variable, in a model with second-order discretization (mostly the default in many physics with COMSOL) you will get a smooth continuous "line". But if your discretization is of first-order (default for some physics) or simply you are plotting a variable that depends on the first spatial derivative of your dependent variable, then this line will have discontinuities or will appear stair-wise/staggered. This is a feature of numerical simulations and the way things are set up here. Another, different issue one might have with cut lines, or planes, is that if your cut domains are discontinuous you cannot get a plot out with the default Plot settings, then you need to change the "x" scale expression from the default line "s variable" to one of the coordinate variables such as x, y or z or any expression thereof.

Regards

From plot setting you can change solid line instead of dots