Fixes and new features for YPane

[realspinner]

Hi, I tried to use RadarChart in a project and faced some problems with scaling, positioning of labels etc. The main problem was the non-configurable offset and turning the plot inside-out when datapoints fell below low Y-bound. I fixed it, here are the changes.

1. Offset is now a property, it can be changed or disabled by setting to 0.0
2. Overlay rendering fixed, it can be used to estimate values now.
3. Mid-point label added for easier estimation of plotted values.
4. All values below the low bound limit are "grounded" to zero now, the plot is not turning inside-out.
5. Origin of the RadarChart, i.e. ray for data point with zero index now can be rotated to an arbitrary angle. It is helpful when the chart must be drawn upside-down for example. A new property, originAngle has been introduced.