Negative values are not shown in pie chart

[jjmontesl]

From #100, negative values are not shown by the pie chart.

Possibilities:

1. At least inform the user that N slices are missing due to negative values
2. Shown as 0 values to force their inclusion in the legend.
3. Show it correctly if all of them are negative.

[62mkv]

I would say all 3 options are non-exclusive and could be implemented, or at least I like it and that would be a straight-forward user experience, I think