RE: How to vary color in an area chart between negative or positive

The area chart has 2 data series. 
The negative series is created directly from the original data. 
The positive data series is derived via formula from the original data and plotted on the secondary x-axis.

You can not simply use 2 series plotting on positive or negative values as this do not create the same x-axis crossing points as a single set of data.

The formula for the positive series calculates the x-axis crossing points. To plot these values correctly the secondary axis is changed to a date value rather than category value axis. In this example I used 100 days between category points.

This workbook contains an example.