bbox_inches='tight' error, is this an mpl bug or misuse on my part?

Hi all,

I'm getting an error (with current mpl master) illustrated by this code:

···

###
from cStringIO import StringIO

import matplotlib.pyplot as plt
import matplotlib.lines as lines
fig = plt.figure()
l1 = lines.Line2D([0, 1], [0, 1], transform=fig.transFigure, figure=fig)
l2 = lines.Line2D([0, 1], [1, 0], transform=fig.transFigure, figure=fig)
fig.lines.extend([l1, l2])
fig.canvas.draw()

sio = StringIO()
fig.canvas.print_figure(sio, format='png', bbox_inches='tight')
###

Is this a bug, or am I misusing print_figure?

I don't want to open a ticket if it's not a real bug, if it is one
I'll file it on gh.

Thanks,

f