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)

While this hasn't changed as part of this pull, why do we console.error here? Usually we try to fatal on unknown errors as console error tend to be "silent" for most users, leading them to believe nothing went wrong.