%matplotlib inline
from yt.mods import *
import numpy as np

def cell_mass_msun(field, data):
    return data["CellMassMsun"]

# Note the use of "raw" strings in display_name and units.
add_field("CellMassMsunAlias", cell_mass_msun, display_name=r"\rm{Cell\/Mass}", units=r"\rm{M}_\odot")

pf = load("IsolatedGalaxy/galaxy0030/galaxy0030")
ad = pf.h.all_data()
p = PhasePlot(ad, "Density", "Temperature", "CellMassMsunAlias")

plot = p.plots["CellMassMsunAlias"]
plot.axes.text(1e-31, 1e7, "Who put this text on my plot?")

p.show()