Basically, when generating plots with matplotlib, The scale on the y-axis goes into the millions. How do I turn on digit grouping (i.e. so that 1000000 displays as 1,000,000) or turn on the decimal separator?
I don't think there's a built-in function to do this. (That's what i thought after i read your Q; i just checked and couldn't find one in the Documentation).
In any event, it's easy to roll your own.
(Below is a complete example--ie, it will generate an mpl plot with one axis having commified tick labels--although five lines of code are all you need to create custom tick labels--three (including import statement) for the function used to create the custom labels, and two lines to create the new labels and place them on the specified axis.)
# first code a function to generate the axis labels you want
# ie, turn numbers greater than 1000 into commified strings (12549 => 12,549)
import locale
locale.setlocale(locale.LC_ALL, 'en_US')
fnx = lambda x : locale.format("%d", x, grouping=True)
from matplotlib import pyplot as PLT
import numpy as NP
data = NP.random.randint(15000, 85000, 50).reshape(25, 2)
x, y = data[:,0], data[:,1]
fig = PLT.figure()
ax1 = fig.add_subplot(111)
ax1.plot(x, y, "ro")
default_xtick = range(20000, 100000, 10000)
# these two lines are the crux:
# create the custom tick labels
new_xtick = map(fnx, default_xtick)
# set those labels on the axis
ax1.set_xticklabels(new_xtick)
PLT.show()