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).
No, the API doesn't.  This method provides the expected trend data, where the empty data point is inserted by the presenter in the case the API returns a single data point.