QChart how to get points that are displayed while zoomed in

Question

The QChart contains some series.

Is it possible to retrieve the list of points that are currently visible on the chart while the chart is zoomed in ?

Means getting a list/vector of point of the actually displayed points.

---

Digested and simplified from Kuba answer:

QList<QVector<QPointF>> XXX::getDisplayedPoints(QChart *chart)
{
   QList<QVector<QPointF>> result;

   foreach (QAbstractSeries * series, chart->series())
   {
       QVector<QPointF> vector;

       auto inScene = chart->plotArea();
       auto inChart = chart->mapFromScene(inScene);
       auto inChartRect = inChart.boundingRect();
       auto inItem1 = chart->mapToValue(inChartRect.topLeft(), series);
       auto inItem2 = chart->mapToValue(inChartRect.bottomRight(), series);
       QRectF rect = QRectF(inItem1, inItem2).normalized();

       const QVector<QPointF> points = static_cast<QLineSeries*>(series)->pointsVector();
       std::copy_if(points.begin(), points.end(), std::back_inserter(vector),
                    [rect](QPointF const &p) { return rect.contains(p); });

       result.append(vector);
   }

   return result;
}

Can be called from signal

connect(static_cast<QValueAxis *>(m_chart->axisX()), &QValueAxis::rangeChanged, this, &XXX::on_zoomUpdated);

Answer 1

You need to map the coordinates of the opposite corners of the chart's plotArea to the world coordinate system of the series you show, and then iterate the series to extract the points fitting that rectangle.

// https://github.com/KubaO/stackoverflown/tree/master/questionschart-visible-points-52777058
#include <QtCharts>
#include <algorithm>
#include <cmath>

auto seriesRect(QChart *chart, QAbstractSeries *series = nullptr) {
   auto inScene = chart->plotArea();
   auto inChart = chart->mapFromScene(inScene);
   auto inChartRect = inChart.boundingRect();
   auto inItem1 = chart->mapToValue(inChartRect.topLeft(), series);
   auto inItem2 = chart->mapToValue(inChartRect.bottomRight(), series);
   return QRectF(inItem1, inItem2).normalized();
}

auto pointsInRect(QXYSeries *series, const QRectF &rect) {
   QVector<QPointF> result;
   auto const points = series->pointsVector();
   std::copy_if(points.begin(), points.end(), std::back_inserter(result),
                [rect](auto &p) { return rect.contains(p); });
   return result;
}

Rest of the example:

auto data() {
   QVector<QPointF> result;
   for (auto x = 0.; x < 10.; x += 0.1) result.append({x, exp(x)});
   return result;
}

int main(int argc, char *argv[]) {
   QApplication a(argc, argv);
   QWidget ui;
   QVBoxLayout layout(&ui);
   QChartView view1, view2;
   QLabel status;
   layout.addWidget(&view1);
   layout.addWidget(&view2);
   layout.addWidget(&status);
   layout.setMargin(4);

   QLineSeries series;
   series.replace(data());
   auto *chart = view1.chart();
   chart->addSeries(&series);
   view1.setRubberBand(QChartView::RectangleRubberBand);

   QLineSeries subSeries;
   subSeries.setPointsVisible(true);
   auto *subChart = view2.chart();
   subChart->addSeries(&subSeries);

   for (auto *chart : {view1.chart(), view2.chart()}) {
      chart->legend()->hide();
      chart->createDefaultAxes();
      chart->layout()->setContentsMargins(0, 0, 0, 0);
   }

   auto update = [&] {
      auto rect = seriesRect(chart, &series);
      auto const points = pointsInRect(&series, rect);

      status.setText(QStringLiteral("Visible Range: (%1,%2)-(%3,%4)")
                         .arg(rect.left())
                         .arg(rect.top())
                         .arg(rect.right())
                         .arg(rect.bottom()));

      subSeries.replace(points);
      subChart->axisX(&subSeries)->setRange(rect.left(), rect.right());
      subChart->axisY(&subSeries)->setRange(rect.top(), rect.bottom());
   };
   QObject::connect(chart, &QChart::plotAreaChanged, update);
   ui.setMinimumSize(400, 400);
   ui.show();
   return a.exec();
}

For the mapping itself, see this answer.