Introducing optional hints for QXYSeries

I recommend to introduce an API ( f.e one or several enums ) where the user can provide additional information about the characteristics of the series.

• clipping of lines
• spline interpolation/approximation
• finding the value below the mouse position
• ...

can be implemented in a way more efficient way, when knowing that: y = f ( x ) ( or x = g( y ) ) in opposite to unordered points.

It also allows to fill the area with some color between the curve and some baseline ( usually 0.0 ) - what is a common use case of line charts.