Multiple bugs fixed + stackedBar with negative values

Enhancements : 
- StackedBar and HorizontalStackedBar can now have negative values;
Bugs fixed
- Hover info remains on graph (issue #19)
- legend not correct (inverted) for gradient Colors in Bar/Lines chart
- gradient Color not displayed correctly on HorizontalBar when negative values in the data;
- animation on HorizontalBar was not correct in some cases;
- when max value was greater than graphMax, the (Horizontal)StackedBar graph was not correct;