If the axes in g2 is the only axes you have open, you could use:
plot(findall(0,'type','axes'),x,y)
This goes in the callback to the pushbutton.
If there is more than one axes open, you will have to do something else, like give the axes in g2 a tag, say 'myaxes' then using this in FINDALL.
axhandle = findall(0,'tag','myaxes');
plot(axhandle,x,y);
