PNG: support lineMode=slope|staircase|staircase

staircase is not implemented
PNG: don't reserve memory for all points for every line
implement drawAsInfinite

Conflicts:
	png.go
PNG: add comment
Adding support for secondYAxis

This require adding second Y axis support on plotinum, the required
changes can be seen here:
https://code.google.com/r/cashraistlin-plotinum/source/detail?r=f71f5b39b33d58a397250165730a0d2ecf44ecac#