Support negative values on bar chart

[fabrijb]

The bar chart don't draw the bars with negative values. I mean, doesn't support the -xy plane.

[truongphamit]

I have the same question

[argamanza]

I'm experiencing the same issue, the bars starts from 0, the 2 red arrows are pointing to places where there should be bars that mark a negative values.
Any way to display them? Can't find in the examples anything about determining the minimum value.

[aloenobilis]

This has been fixed. See #131