Area chart no longer empty by default.

Closes #1769.

Currently the area chart is empty by default. This happens because an area chart cannot be rendered with less than 2 data points. The default value provided when a chart type is selected is only 1 data point.

This pull request adds a function which throws an error whenever an area chart is rendered with 1 or fewer than 2 data points. An error message is displayed in place of the chart.

![screen shot 2014-11-18 at 6 56 23 pm](https://cloud.githubusercontent.com/assets/3149785/5091946/ab2b99c2-6f54-11e4-93e6-d611496a6f7b.png)