Uses area chart instead of line chart

This addresses #95.

Here are some screenshots for comparison. These charts are from the gcc crate:

![without-area](https://cloud.githubusercontent.com/assets/965436/5572139/f4c4ce86-8f67-11e4-8f0c-91ed0b6ab008.png)

![with-area](https://cloud.githubusercontent.com/assets/965436/5572142/f824d71a-8f67-11e4-9078-99c47f72eea4.png)

Ultimately this has a lot to do with personal preference so I understand if the line charts are preferred over the area charts. This ended up being a one line fix so no big deal either way.