Entirely black image in thunar and ristretto image viewer

The examples on the website produce an entirely black images as shown in thunar (xfce) and ristretto image viewer; in iceweasel the images are charts are visible. As an example, the following code does this:

```
import pygal # First import pygal
bar_chart = pygal.Bar() # Then create a bar graph object
bar_chart.add('Fibonacci', [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]) # Add some values
bar_chart.render_to_file('bar_chart.svg') # Save the svg to a file
```

This looks related to #7. I'm not sure if the problem is with pygal or the other applications.

Reproduce the example from the issue and inspect the generated bar_chart.svg in Thunar, Ristretto, and Iceweasel. Read the discussion in issue #7 and compare the viewer results to determine whether the problem is in pygal's SVG output or the applications; done means the cause is identified and the affected rendering behavior is corrected or documented.

Written by the indexing model from the issue text.