The following script gives me different results in the 2d phase plot when I use multiple cores and switch the fieldc parameter.  The results are consistent if I only use a single core.  Some of the plots produced seems like it is missing data points or under-sampled, so I'm guessing maybe not all the data from all the cores were collected at the end.

From
G.S.

#-----------------------------------------------------------------------------------
from yt.mods import *

i=2
file = "RD%04i" % i
fn = file+"/"+file
pf=load(fn)
dd=pf.h.all_data()

fieldx='Density'
fieldy='Temperature'
fieldc='CellVolumeCode'
#fieldc='CellVolume'                                                            
#fieldc='CellMassMsun'                                                          
#fieldc='CellMass'                                                              
#fieldc='Ones'                                                                  

pplot = PhasePlot(dd, fieldx, fieldy, [fieldc],weight_field=None)
pplot.save('example/'+file)
#---------------------------------------------------------------------------------------