RECTF = RBBOX(RECTI, XY) tracks a rubberband box in the current
figure, initialized with box RECTI, and starting the tracking at
the point XY. The mouse button must be help down when RBBOX is
called. RBBOX returns with the mouse up button event. As a
result, RBBOX can be used in an M-file, along with
WAITFORBUTTONPRESS, to control dynamic behavior.
RECTF is the rect specified by the initial point of RECTI and its
extent is determined by where the mouse up event occurs.
RECTI, XY, RECTF are in units of the current figure.