fix galactocentric example and add plot [docs only]
fix galactocentric example and add plot [docs only]

funbaker/astropy,saimn/astropy,mhvk/astropy,StuartLittlefair/astropy,saimn/astropy,lpsinger/astropy,stargaser/astropy,pllim/astropy,AustereCuriosity/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,saimn/astropy,aleksandr-bakanov/astropy,DougBurke/astropy,dhomeier/astropy,MSeifert04/astropy,DougBurke/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,dhomeier/astropy,stargaser/astropy,astropy/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,dhomeier/astropy,lpsinger/astropy,MSeifert04/astropy,bsipocz/astropy,astropy/astropy,dhomeier/astropy,AustereCuriosity/astropy,stargaser/astropy,funbaker/astropy,bsipocz/astropy,pllim/astropy,aleksandr-bakanov/astropy,mhvk/astropy,aleksandr-bakanov/astropy,DougBurke/astropy,astropy/astropy,pllim/astropy,pllim/astropy,mhvk/astropy,DougBurke/astropy,astropy/astropy,larrybradley/astropy,funbaker/astropy,funbaker/astropy,lpsinger/astropy,saimn/astropy,MSeifert04/astropy,saimn/astropy,bsipocz/astropy,AustereCuriosity/astropy,bsipocz/astropy,AustereCuriosity/astropy,MSeifert04/astropy,StuartLittlefair/astropy,dhomeier/astropy,astropy/astropy,mhvk/astropy,stargaser/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,lpsinger/astropy