Paste: plot demo

Hello,

A simple plot gadget has been checked in. You can try this to see a
demonstration of it:

       USE: ui.gadgets.plot

       <plot> [ sin ] add-function dup gadget.

Move the mouse over the gadget. The arrow keys will change the view. 'a'
and 'z' zoom in and out. Beware; the focus is taken by the plot gadget when
you move the mouse into it so if you want to type into the listener input
area, you'll have to click on it.

You should still have the plot gadget on the stack. Add another function with
color:

       [ cos 3 * ] blue function boa add-function

Ed