Percentage stacked area with negative values is incorrect

[TorsteinHonsi]

Demo: http://jsfiddle.net/highcharts/jBpcT/

The negative value should be substracted from the stack rather than added and displayed outside the plot area.

For reference, this is how Excel displays the same data:

[yulinwang]

Two issues remaining for stacked percentage chart after this fix:

1. Multiple negative values on Area chart will have blank area.
   This is sample: http://jsfiddle.net/LionelWang/m59xj/6/.
2. Negative values in Column chart are ignored.
   This is sample: http://jsfiddle.net/LionelWang/m59xj/7/
   Could you fix these as well?

Thanks

[TorsteinHonsi]

Reference images for the two cases, from Excel: