/*!
     \example demos/callout
     \title Callout demo
     \subtitle

     This demo shows how to draw an additional element (a callout) on top of the chart.
     \image demos_callout.png

     QChart class provides two methods that map between the scene coordinates and the series' domain (defines by the axes ranges).
     QPointF QChart::mapToPosition(const QPointF &value, QAbstractSeries *series)
     QPointF QChart::mapToValue(const QPointF &position, QAbstractSeries *series)
*/