One way to do it is to reset the callback 'windowbuttondownfcn' after two clicks. Something like this:
function pushbutton1_Callback(hObject, eventdata, handles)
global numClicks
numClicks = 0;
set(gcf, 'windowbuttondownfcn', @Draw0);
-----
function Draw0(src,evnt)
global x0 y0 numClicks
pt = get(gca, 'CurrentPoint');
x0 = pt(1, 1);
y0 = pt(1, 2);
numClicks= numClicks+1;
hold on
plot(x0,y0,'o','MarkerEdgeColor','k',...
'MarkerFaceColor','r',...
'MarkerSize',8)
if numClicks > 2,
set(src, 'windowbuttondownfcn', '');
end
