How to export plots from matplotlib with transparent background?

I am using matplotlib to make some graphs and unfortunately I cannot export them without the white background.

sample plot with solid white background

In other words, when I export a plot like this and position it on top of another image, the white background hides what is behind it rather than allowing it to show through. How can I export plots with a transparent background instead?

Use the matplotlib savefig function with the keyword argument transparent=True to save the image as a png file.

    In [30]: x = np.linspace(0,6,31)

    In [31]: y = np.exp(-0.5*x) * np.sin(x)

    In [32]: plot(x, y, 'bo-')
    Out[32]: [<matplotlib.lines.Line2D at 0x3f29750>]            

    In [33]: savefig('demo.png', transparent=True)

Result: demo.png

Of course, that plot doesn't demonstrate the transparency. Here's a screenshot of the PNG file displayed using the ImageMagick display command. The checkerboard pattern is the background that is visible through the transparent parts of the PNG file.

display screenshot