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).
In the case that there were few points (e.g. two points), the chart looked very odd with only a start date as the last tick was removed.  By deleting this line, we get at both start and end tick marks.  I didn't see any display issues with larger datasets too.  Are there concerns?