This is a handy trick for unit tests and the like, when you need to do a pixel-to-pixel comparison with a saved plot.

One way is to use `fig.canvas.tostring_rgb`

and then `numpy.fromstring`

with the approriate dtype. There are other ways as well, but this is the one I tend to use.

E.g.

```
import matplotlib.pyplot as plt
import numpy as np
# Make a random plot...
fig = plt.figure()
fig.add_subplot(111)
# If we haven't already shown or saved the plot, then we need to
# draw the figure first...
fig.canvas.draw()
# Now we can save it to a numpy array.
data = np.frombuffer(fig.canvas.tostring_rgb(), dtype=np.uint8)
data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))
```