create saved line in foreground, move to background when interactive line changes, #36
create saved line in foreground, move to background when interactive line changes, #36

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/graphing-quadratics,phetsims/graphing-quadratics,phetsims/graphing-quadratics