Log Message:
V 1.5.1
Add `stack_from_top` option to reverse stack graph data order
Minor fix for empty logarithmic chart
Reorders axes in SVG output. Fix #145 (thanks sirlark)

V 1.5.0
Add per serie configuration
Add half pie (thanks philt2001)
Make lxml an optionnal dependency (huge speed boost in pypy)
Add render_table (WIP)
Support colors in rgb / rgba for parametric styles