You can mask some data in a curve by using the mouse with the ⌥ key. When you hold the ⌥ key inside the plotting area (the plot frame) of a plot view, clicks draws a polygon. All points of all curves outside of this polygon will be masked.

• To end selection, double-click in the view. To cancel, press esc.
• An empty selection unmasks all data.

Fits made with the Graphics > Fit curve menu will take masks into account.