Function: plotkill
Section: graphic
C-Name: plotkill
Prototype: vL
Help: plotkill(w): erase the rectwindow w.
Doc: erase rectwindow $w$ and free the corresponding memory. Note that if you
 want to use the rectwindow $w$ again, you have to use \kbd{plotinit} first
 to specify the new size. So it's better in this case to use \kbd{plotinit}
 directly as this throws away any previous work in the given rectwindow.
