Plotting negative numbers

[ghost]

Plotting negative numbers doesn't seem to be well supported.

Building a bar or a line visualisation based on negative numbers (using a "Min" or "Average" for example) works but the minus sign is simply ignored.
A visualisation based on negative numbers only looks exactly the same as a visualisation build on positive numbers only. No minus symbol on the y axis or anywhere else (apart on the mousover).
I don't know exactly what happen with a visualisation with a mix of positive and negative numbers, but I suspect the sign will be ignored, making the visualisation plain wrong.

[rashidkpc]

Duplicate of #1962