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).
If that's the case, please rename this function to make it clear that the returned value is data expected from the presenter, not the API method.