better logic in x_to_pix
better logic in x_to_pix

vlas-sokolov/pyspeckit,keflavich/pyspeckit,pyspeckit/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit,pyspeckit/pyspeckit,keflavich/pyspeckit,e-koch/pyspeckit,jpinedaf/pyspeckit,low-sky/pyspeckit,vlas-sokolov/pyspeckit,allisony/pyspeckit,e-koch/pyspeckit,low-sky/pyspeckit