Reducing size of vectorized contourplot

Question

I would like to include a filled contour plot to a pdf document (for example a TeX document). Currently I am using `pyplot`s `contourf`, and saving to `pdf` with `pyplot`s `savefig`. The problem with this is that the size of the plots becomes rather big as compared to a high resolution `png`. One way to reduce the size is of course to reduce the number of levels in the plot, but too few levels gives a poor plot. I'm searching for a simple way to for example let the colors of the plot be saved as a png, with the axes, ticks etc. to be saved vectorized.

Accepted Answer

You can do this using the `Axes` option `set_rasterization_zorder`.

Anything with a `zorder` less than what you set that to be will be saved as rasterized graphics, even when saving to a vector format like `pdf`.

For example:

```
import matplotlib.pyplot as plt
import numpy as np

data = np.random.rand(500,500)

# fig1 will save the contourf as a vector
fig1,ax1 = plt.subplots(1)
ax1.contourf(data)
fig1.savefig('vector.pdf')

# fig2 will save the contourf as a raster
fig2,ax2 = plt.subplots(1)
ax2.contourf(data,zorder=-20)
ax2.set_rasterization_zorder(-10)
fig2.savefig('raster.pdf')

# Show the difference in file size. "os.stat().st_size" gives the file size in bytes.
print os.stat('vector.pdf').st_size
# 15998481
print os.stat('raster.pdf').st_size
# 1186334
```

You can see this matplotlib example for more background info.

As pointed out by @tcaswell, to rasterise just one artist without having to affect its `zorder`, you can use `.set_rasterized`. However, this doesn't appear to be an option with `contourf`, so you would need to loop over the `PathCollections` created by `contourf` and `set_rasterized` on each of them. Something like this:

```
contours = ax.contourf(data)
for pathcoll in contours.collections:
    pathcoll.set_rasterized(True)
```

Reducing size of vectorized contourplot

Solution

Problem