Below is the documentation for BasePlot. You plot a xy line and then supply a BasePlot along with it and color the space in between creating an envelope.
ScottyO
CNiPlot BasePlot;
Description
Specifies a plot to use for Y values when FillToBase or LineToBase is enabled.
Note: Points in the plot are paired with points in the base plot. Filling or drawing lines between plots works best when the X coordinates for each pair are the same.
Ok, I see. This was added to the .NET graph in Measurement Studio 7.1. To do this with the .NET graphs, go to the plot and set its BasePlot property to another plot, set the FillBase property to Plot, and set the FillMode property to Fill.