将一个QCPGraph叠加到另一个QCPGraph上显示，就得到了折线堆叠图

static QCPScatterStyle GetScatterStyle(const QColor& color)
{
    QPen pen(color, 2);
    return QCPScatterStyle(
            QCPScatterStyle::ssCircle,
            pen,
            Qt::white, 5);
}

static QCPGraph* AddGraph(
        QCustomPlot* parentPlot,
        const QVector<double>& keys,
        const QVector<double>& values,
        const QColor& color,
        const QString& name)
{
    auto graph = parentPlot->addGraph();
    graph->setData(keys, values);
    graph->setScatterStyle(GetScatterStyle(color));
    graph->setPen(QPen(QColor(color), 2));
    graph->setName(name);
    return graph;
}

void StackedLineChart::initCustomPlot(QCustomPlot *parentPlot)
{
    QVector<double> x = {
            1, 2, 3, 4, 5, 6, 7,
    };
    QVector<double> y = {
            150, 230, 224, 218, 135, 147, 260
    };

    QVector<QString> labels = {
            "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"
    };
    auto ticker = QSharedPointer<QCPAxisTickerText>::create();
    ticker->setTicks(x, labels);

    QPen pen(QColor("#5c7bd9"), 2);
    QCPScatterStyle style(
            QCPScatterStyle::ssCircle,
            pen,
            Qt::white, 5);

    auto graph1 = AddGraph(parentPlot, x, {120, 132, 101, 134, 90, 230, 210},
                           "#5c7bd9", "Email");
    auto graph2 = AddGraph(parentPlot, x, {220, 182, 191, 234, 290, 330, 310},
                           "#91cc75", "Union Ads");
    auto graph3 = AddGraph(parentPlot, x, {150, 232, 201, 154, 190, 330, 410},
                           "#fac858", "Video Ads");
    auto graph4 = AddGraph(parentPlot, x, {320, 332, 301, 334, 390, 330, 320},
                           "#ee6666", "Direct");
    auto graph5 = AddGraph(parentPlot, x, {820, 932, 901, 934, 1290, 1330, 1320},
                           "#73c0de", "Search Engine");

    graph5->moveAbove(graph4);
    graph4->moveAbove(graph3);
    graph3->moveAbove(graph2);
    graph2->moveAbove(graph1);

    parentPlot->xAxis->setTicker(ticker);
    parentPlot->xAxis->grid()->setVisible(false);
    parentPlot->xAxis->setRange(0, 8);
    parentPlot->yAxis->setRange(0, 3000);
    parentPlot->legend->setVisible(true);
    parentPlot->legend->setBorderPen(Qt::NoPen);
    parentPlot->legend->setFillOrder(QCPLayoutGrid::foColumnsFirst);
    parentPlot->axisRect()->insetLayout()->setInsetAlignment(0, Qt::AlignHCenter | Qt::AlignTop);
}