QwPlottMultiBarChart determining legend title of bar segment under cursor

[Uwe]

I've successfully accessed the QwtPlotMultiBarChart::legendData & I see all my legend titles.

So far you did what I would recommend - beside that you could also use QwtPlotMultiBarChart::barTitles() directly.

My problem is determining what section of the bar is under the cursor. I thought at first I could loop through the series data and find what is under the cursor based on the x and y coordinates but this doesn't work since sometimes the multi bars are stacked and sometimes they are grouped.

As you already found a way to identify the chart below the mouse you can easily identify if its current state is Grouped or Stacked. Not sure why this is a problem - beside, that you need different implementations for both modes ?

But maybe it is a better idea to introduce a lookup table with the geometries of each bar:

class YourChart: public QwtPlotMultiBarChart
{
    virtual void draw( QPainter *painter,
        const QwtScaleMap &xMap, const QwtScaleMap &yMap,
        const QRectF &canvasRect ) const
    {
        m_barGeometries.clear();
        QwtPlotMultiBarChart::draw( painter, xMap, yMap, canvasRect );
    }
 
    virtual void drawBar( QPainter *painter,
        int sampleIndex, int valueIndex, const QwtColumnRect &rect ) const
    {
       m_barGeometries.store( sampleIndex, valueIndex, rect );
       QwtPlotMultiBarChart::drawBar( painter, sampleIndex, valueIndex, rect );
    }
 
    QwtText barTitleAt( QPoint &pos ) const
    {
        // find sampleIndex/valueIndex from m_barGeometries
        // and return the title using barTitles()
        ...
    }
private:
    LookUpTable m_barGeometries;
};

class YourChart: public QwtPlotMultiBarChart
{
    virtual void draw( QPainter *painter,
        const QwtScaleMap &xMap, const QwtScaleMap &yMap,
        const QRectF &canvasRect ) const
    {
        m_barGeometries.clear();
        QwtPlotMultiBarChart::draw( painter, xMap, yMap, canvasRect );
    }

    virtual void drawBar( QPainter *painter,
        int sampleIndex, int valueIndex, const QwtColumnRect &rect ) const
    {
       m_barGeometries.store( sampleIndex, valueIndex, rect );
       QwtPlotMultiBarChart::drawBar( painter, sampleIndex, valueIndex, rect );
    }

    QwtText barTitleAt( QPoint &pos ) const
    {
        // find sampleIndex/valueIndex from m_barGeometries
        // and return the title using barTitles()
        ...
    }
private:
    LookUpTable m_barGeometries;
};

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As you need widget coordinates fro barTitleAt() you would have to overload QwtPlotPicker::trackerText() instead of QwtPlotPicker::trackerTextF().

Uwe