Moving a signal

To move a signal from one plot to another, edit its Style so that its Plot number corresponds to that of the target plot. 

For example, if you have read two signals with the default settings into a previously empty program, they will have been given plot numbers 1 and 2.  To plot them on the same axes, select the second one, click 'Style...' on the View/Print window, change the Plot number (top left) to 1, and click 'Exit'.

More details of styles and changing how a signal is plotted are given here.