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 concerns. It simply was not clear that you combined the axis clipping and minimal data tasks.