You can use QGraphicsView to render your QChart as well as any other customized QGraphicsItem.
You have first to subclass QGraphicsView, within the constructor, you can call the virtual method scene() .
Which would you able add QCharts and other QGraphicsItem objects. i.e. scene()->addItem(new QChart) or scene()->addItem(new QGraphicsItem).

And for the coordinates of QChart, I am not quite sure, but you might subclass a QGraphicsItem parent it to the QChart and override it's paint method

void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget);

to draw custom shapes within the QCharts coordinate system using QPainter.