Matplotlib hangs due to numpy.ma.power()

Hall�chen!

I've had a very odd observation today: I call matplotlib 1.3.1 from
Django 1.7 code running on Apache under Ubuntu 14.04. The relevant
code is:

    figure = Figure(frameon=False, figsize=(4, 3))
    canvas = FigureCanvasAgg(figure)
    axes = figure.add_subplot(111)
    ...
    canvas.print_figure(plot_filepath)

The last command hangs.

Funny enough, if I run the very same code on Django's testserver, it
works.

First, I thought of http://stackoverflow.com/questions/22337250 but
this is not the case here. I've really assured that "Agg" is used,
also by providing a proper config file.

The oddest part is, if I replace in matplotlib/scale.py, line 136,

        return ma.power(10.0, a) / 10.0

with

        return np.power(10.0, a) / 10.0

it works!

Can anybody explain this?

Regards,
Torsten.

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