Select the curve type for the active plot type. For more information see the Curve type section under Plot->Layout->Advanced….
If is checked, the curve belonging to the active measurement will be displayed with a width different from the other curves. If curves from several measurements are plotted, this may be useful to emphasize the active curve as shown below.
Note: If a curve width greater than 1 is used, the plotting speed is decreased. So as a faster alternative to this option is to use Plot->Solo Curve.