Add the possibility to create custom QAbstractSeries

For the moment it's impossible to create a custom chart serie or derive from other one for some reasons :

• The enum QAbstractSeries::SeriesType don't have a UserType defined
• The XYChart class is private so it's impossible to derive from it in order to create a custom displayed item

Could you add the possibility to create custom chart series ?

For example, in my case I would like to have a spline serie with control points customisable.

As QSplineSeries cannot user define control points, I would like to create my own serie instead based on QLineSeries.