- Qt 6.12
- Qt Charts
- Creating a Donut Breakdown Chart
- Qt Charts | Commercial or GPLv3

# Creating a Donut Breakdown Chart

Note: This is part of the Charts with Widgets Gallery example.

## Creating Donut Breakdown Charts

Let's start by defining some data for the chart.

     // Graph is based on data of 'Total consumption of energy increased by 10 per cent in 2010'
     // Statistics Finland, 13 December 2011
     // http://www.stat.fi/til/ekul/2010/ekul_2010_2011-12-13_tie_001_en.html

     auto series1 = new QPieSeries;
     series1->setName("Fossil fuels");
     series1->append("Oil", 353295);
     series1->append("Coal", 188500);
     series1->append("Natural gas", 148680);
     series1->append("Peat", 94545);

     auto series2 = new QPieSeries;
     series2->setName("Renewables");
     series2->append("Wood fuels", 319663);
     series2->append("Hydro power", 45875);
     series2->append("Wind power", 1060);

     auto series3 = new QPieSeries;
     series3->setName("Others");
     series3->append("Nuclear energy", 238789);
     series3->append("Import energy", 37802);
     series3->append("Other", 32441);

Then we create a chart where we add the data. Note that this is our own chart derived from QChart.

     auto donutBreakdown = new DonutBreakdownChart;
     donutBreakdown->setAnimationOptions(QChart::AllAnimations);
     donutBreakdown->setTitle("Total consumption of energy in Finland 2010");
     donutBreakdown->legend()->setAlignment(Qt::AlignRight);
     donutBreakdown->addBreakdownSeries(series1, Qt::red);
     donutBreakdown->addBreakdownSeries(series2, Qt::darkGreen);
     donutBreakdown->addBreakdownSeries(series3, Qt::darkBlue);

Our own chart works in such a way that we create a main series in the constructor we create a main series, which aggregates the data provided by the breakdown series. This is the piechart in the center.

     DonutBreakdownChart::DonutBreakdownChart(QGraphicsItem *parent, Qt::WindowFlags wFlags)
         : QChart(QChart::ChartTypeCartesian, parent, wFlags)
     {
         // create the series for main center pie
         m_mainSeries = new QPieSeries;
         m_mainSeries->setPieSize(0.7);
         QChart::addSeries(m_mainSeries);
     }

When a breakdown series is added the data is used to create a slice in the main series and the breakdown series itself is used to create a segment of a donut positioned so that it is aligned with the corresponding slice in the main series.

     void DonutBreakdownChart::addBreakdownSeries(QPieSeries *breakdownSeries, QColor color)
     {
         QFont font("Arial", 8);

         // add breakdown series as a slice to center pie
         auto mainSlice = new DonutBreakdownMainSlice(breakdownSeries);
         mainSlice->setName(breakdownSeries->name());
         mainSlice->setValue(breakdownSeries->sum());
         m_mainSeries->append(mainSlice);

         // customize the slice
         mainSlice->setBrush(color);
         mainSlice->setLabelVisible();
         mainSlice->setLabelColor(Qt::white);
         mainSlice->setLabelPosition(QPieSlice::LabelInsideHorizontal);
         mainSlice->setLabelFont(font);

         // position and customize the breakdown series
         breakdownSeries->setPieSize(0.8);
         breakdownSeries->setHoleSize(0.7);
         breakdownSeries->setLabelsVisible();
         const auto slices = breakdownSeries->slices();
         for (QPieSlice *slice : slices) {
             color = color.lighter(115);
             slice->setBrush(color);
             slice->setLabelFont(font);
         }

         // add the series to the chart
         QChart::addSeries(breakdownSeries);

         // recalculate breakdown donut segments
         recalculateAngles();

         // update customize legend markers
         updateLegendMarkers();
     }

Here's how the start and end angles for the donut segments are calculated.

     void DonutBreakdownChart::recalculateAngles()
     {
         qreal angle = 0;
         const auto slices = m_mainSeries->slices();
         for (QPieSlice *slice : slices) {
             QPieSeries *breakdownSeries = qobject_cast<DonutBreakdownMainSlice *>(slice)->breakdownSeries();
             breakdownSeries->setPieStartAngle(angle);
             angle += slice->percentage() * 360.0; // full pie is 360.0
             breakdownSeries->setPieEndAngle(angle);
         }
     }

The legend markers are customized to show the breakdown percentage. The markers for the main level slices are hidden.

     void DonutBreakdownChart::updateLegendMarkers()
     {
         // go through all markers
         const auto allseries = series();
         for (QAbstractSeries *series : allseries) {
             const auto markers = legend()->markers(series);
             for (QLegendMarker *marker : markers) {
                 auto pieMarker = qobject_cast<QPieLegendMarker *>(marker);
                 if (series == m_mainSeries) {
                     // hide markers from main series
                     pieMarker->setVisible(false);
                 } else {
                     // modify markers from breakdown series
                     pieMarker->setLabel(QString("%1 %2%")
                                         .arg(pieMarker->slice()->label())
                                         .arg(pieMarker->slice()->percentage() * 100, 0, 'f', 2));
                     pieMarker->setFont(QFont("Arial", 8));
                 }
             }
         }
     }

Instead the main level slices show the percentage on the label.

     DonutBreakdownMainSlice::DonutBreakdownMainSlice(QPieSeries *breakdownSeries, QObject *parent)
         : QPieSlice(parent),
           m_breakdownSeries(breakdownSeries)
     {
         connect(this, &DonutBreakdownMainSlice::percentageChanged,
                 this, &DonutBreakdownMainSlice::updateLabel);
     }
         ...
     void DonutBreakdownMainSlice::updateLabel()
     {
         setLabel(QString("%1 %2%").arg(m_name).arg(percentage() * 100, 0, 'f', 2));
     }

Now that we have our chart defined, we can finally create a QChartView and show the chart.

     createDefaultChartView(donutBreakdown);