2ndary axis

Addition of secondary y-axis to Line graphs - series might be drawn on either Y-axis, which may have different scales.

Changes:
- legend box is always on the left instead right for primary series
- secondary series legend is on the right

Apart from that - not extracted separatly - there is a change to tooltips, to present X label in tooltip, which works nicely with limitation of x labels count.

I'm not sure, whether I shall use another View instance to do proper scaling of secondary series instead.