At the bottom of the main window are a pair of parameters which affect how that group is plotted. The «plot multiplier» parameter is a multiplicative factor for the group when it gets plotted. The «y-axis offset» is a number added to the group when it is plotted so that it is offset in the vertical direction. Both of these are parameters that are set for each group individually.
Stacked plots are made by setting the «y-axis offset» parameters for many groups.
Like most other parameters in the main window, context menus accessed by right clicking on the labels can be used to constrain these parameters across data groups.