From ift

To create a butterfly curve, for example for characterizing SRAM read and hold margins, simulate a single DC sweep (one source, one sweep) on a input. Plot both the input (shows up as a straight line for a linear DC sweep) and the output (shows up as the DC response of your circuit).

Have both plots in the same subwindows.

go to Axis -> Y vs Y choose the first trace, press ok.

go again to Axis -> Y vs Y choose the second trace, press ok.

Put both results in the same plot, and you are finished.