Highcharts Bug Fix with date display and tool tips

This references https://github.com/cbmi/harvest-openmrs/issues/20

The threshold in the options needed to be adjusted. Also, in order to display the values properly on the tooltip, use the formatter. 
![screen shot 2014-06-02 at 11 35 03 am](https://cloud.githubusercontent.com/assets/2774472/3149526/2e9d9026-ea6c-11e3-9b96-a339c9207751.png)