bugfix: (questionable whether this is a fix): force the order of plotted data to be ascending
bugfix: (questionable whether this is a fix): force the order of plotted
data to be ascending

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