Display empty previous data point on trend line when only one data point exists.

@rocha @clintonb @marcotuts 

FYI: @shnayder 

Example of engagement trend display with empty data point:
![screen shot 2014-10-06 at 1 39 43 pm](https://cloud.githubusercontent.com/assets/5265058/4530325/c8cf8cbe-4d7f-11e4-8c18-8c17f0e120db.png)

Please note that the data table includes this empty data (not in CSV download).
Does the API actually return a zero enrollment data point?