Question about step point

I collected the coordinates of each step and then visualized them,and finally want to draw such a picture as:

But the result is:

I considered setting the max step length

        G4double maxStep = 0.05*um;
        auto fStepLimit = new G4UserLimits(maxStep);

which result in

The trajectory of each particle in the figure always seems to have a fixed distance on the x-axis.