function undo(N)%UNDO   Erase the last object plotted on the graph.  The object can%       be a curve or text.  Repeating "undo" erases further back in%       time. UNDO does not work for title and labels; to erase%       these, use "title('')" or "xlabel('')"   %       kwong@mcs.anl.gov 1/19/93
 % 		Modified A. Knight, April 1999 
h = get(gca,'children');
if length(h) > 0    if nargin==0	  delete(h(1))	else	  delete(h(1:N));  end
end