Add

Click here to enable the interactive addition of connecting line
segments.  As you move the cursor in the plot window, the nearest
point is marked with a diamond.  To add a line between two points,
press and hold the left button when one of the points is the nearest
point.  As you drag the cursor, a dotted line connects that point to
the nearest point.  When the dotted line is where you want the final
line to me, release the mouse button.

If you realize that you didn't want that point after all, then click
middle and it will be deleted.