Rubberband box.

  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.