创建极坐标图表

它还展示了如何实现极坐标图的滚动和缩放，以及直观地演示极坐标图表和笛卡尔图表之间的关系。

创建极坐标图使用 QPolarChart 实例，而不是 QChart 实例。

auto chart = new QPolarChart;

轴的创建与笛卡尔图表类似，但当轴添加到图表中时，您可以使用极坐标方向而不是对齐方式。

auto angularAxis = new QValueAxis;
angularAxis->setTickCount(9); // First and last ticks are co-located on 0/360 angle.
angularAxis->setLabelFormat("%.1f");
angularAxis->setShadesVisible(true);
angularAxis->setShadesBrush(QBrush(QColor(249, 249, 255)));
chart->addAxis(angularAxis, QPolarChart::PolarOrientationAngular);

auto radialAxis = new QValueAxis;
radialAxis->setTickCount(9);
radialAxis->setLabelFormat("%d");
chart->addAxis(radialAxis, QPolarChart::PolarOrientationRadial);

极坐标图的缩放和滚动在逻辑上与笛卡尔图表的缩放和滚动几乎相同。主要区别在于沿 X 轴（角度轴）滚动时，使用角度而不是像素数。另一个区别是无法缩放到矩形。

void PolarChartView::keyPressEvent(QKeyEvent *event)
{
    switch (event->key()) {
    case Qt::Key_Plus:
        chart()->zoomIn();
        break;
    case Qt::Key_Minus:
        chart()->zoomOut();
        break;
    case Qt::Key_Left:
        chart()->scroll(-1.0, 0);
        break;
    case Qt::Key_Right:
        chart()->scroll(1.0, 0);
        break;
    case Qt::Key_Up:
        chart()->scroll(0, 1.0);
        break;
    case Qt::Key_Down:
        chart()->scroll(0, -1.0);
        break;
    case Qt::Key_Space:
        switchChartType();
        break;
    default:
        QGraphicsView::keyPressEvent(event);
        break;
    }
}

相同的轴和序列可用于笛卡尔坐标和极坐标图表，尽管不能同时使用。要在图表类型之间切换，您首先需要从旧图表中移除序列和轴，然后将它们添加到新图表。如果想要保留轴范围，还需要复制这些范围。

void PolarChartView::switchChartType()
{
    QChart *newChart;
    QChart *oldChart = chart();

    if (oldChart->chartType() == QChart::ChartTypeCartesian)
        newChart = new QPolarChart;
    else
        newChart = new QChart;

    // Move series and axes from old chart to new one
    const QList<QAbstractSeries *> seriesList = oldChart->series();
    const QList<QAbstractAxis *> axisList = oldChart->axes();
    QList<QPair<qreal, qreal> > axisRanges;

    for (QAbstractAxis *axis : axisList) {
        auto valueAxis = static_cast<QValueAxis *>(axis);
        axisRanges.append(QPair<qreal, qreal>(valueAxis->min(), valueAxis->max()));
    }

    for (QAbstractSeries *series : seriesList)
        oldChart->removeSeries(series);

    for (QAbstractAxis *axis : axisList) {
        oldChart->removeAxis(axis);
        newChart->addAxis(axis, axis->alignment());
    }

    for (QAbstractSeries *series : seriesList) {
        newChart->addSeries(series);
        for (QAbstractAxis *axis : axisList)
            series->attachAxis(axis);
    }

    int count = 0;
    for (QAbstractAxis *axis : axisList) {
        axis->setRange(axisRanges[count].first, axisRanges[count].second);
        count++;
    }

    newChart->setTitle(oldChart->title());
    setChart(newChart);
    delete oldChart;
}