在地震剖面中，我们常需要绘制如下的波形图，如何使用Qt来实现呢？下面是一种尝试，可能帮你解决问题。如果你有更好的方案，欢迎评论，帮助更多人解决问题。

单道波形图

方法1： 将点集划分为x轴上方和下方的点，对于x轴交点位置，需要求解。使用直线方程求解。

    customPlot->addGraph();
    customPlot->graph(0)->setPen(Qt::NoPen); // no line
    customPlot->graph(0)->setBrush(QBrush(QColor(0, 0, 0, 255))); // first graph will be filled with black

    customPlot->addGraph();
    customPlot->graph(1)->setPen(QPen(Qt::black)); // only draw line under x-axis
    // customPlot->graph(1)->setBrush(QBrush(QColor(0, 255, 0, 255)));

    // generate some points of data (y0 for first, y1 for second graph):
    QVector<double> x(1000), y0(1000);
    for (int i = 0; i < 1000; ++i) {
            x[i] = i / 10.0;
            y0[i] = sin(x[i]) * exp(-x[i] / 10.0) + 0.1 * (rand() / (double)RAND_MAX - 0.5);
    }

    QVector<double> xPositive, yPositive, xNegative, yNegative;
    for (int i = 0; i < x.size() - 1; ++i) {
        if(std::abs(y0[i]) < 1e-9){
            xPositive.push_back(x[i]);
            xNegative.push_back(x[i]);
            yPositive.push_back(y0[i]);
            yNegative.push_back(y0[i]);
            continue;
        }

        bool isCrossing = y0[i] * y0[i + 1] <= 0;
        if (y0[i] > 0) {
            xPositive.push_back(x[i]);
            yPositive.push_back(y0[i]);
        } else if(y0[i] < 0){
            xNegative.push_back(x[i]);
            yNegative.push_back(y0[i]);

        }

        if (isCrossing) {
            double xCross = -1.0 * (x[i] * y0[i + 1] - x[i + 1] * y0[i]) / (y0[i] - y0[i + 1]);
            xPositive.push_back(xCross);
            yPositive.push_back(0);
            xNegative.push_back(xCross);
            yNegative.push_back(0);
        }
    }

     if(y0.back() >= 0){
         xPositive.push_back(x.back());
         yPositive.push_back(y0.back());
     }

     if(y0.back() <= 0){
         xNegative.push_back(x.back());
         yNegative.push_back(y0.back());
     }

     customPlot->xAxis2->setVisible(true);
     customPlot->xAxis2->setTickLabels(false);
     customPlot->yAxis2->setVisible(true);
     customPlot->yAxis2->setTickLabels(false);
     // make left and bottom axes always transfer their ranges to right and top axes:
     connect(customPlot->xAxis, SIGNAL(rangeChanged(QCPRange)), customPlot->xAxis2, SLOT(setRange(QCPRange)));
     connect(customPlot->yAxis, SIGNAL(rangeChanged(QCPRange)), customPlot->yAxis2, SLOT(setRange(QCPRange)));
     // pass data points to graphs:
     customPlot->graph(0)->setData(xPositive, yPositive);
     // customPlot->graph(0)->setData(x, y0);
     customPlot->graph(1)->setData(xNegative, yNegative);
     // customPlot->graph(2)->setData({0, 251}, {0, 0});
     // let the ranges scale themselves so graph 0 fits perfectly in the visible area:
     customPlot->graph(0)->rescaleAxes();
     // same thing for graph 1, but only enlarge ranges (in case graph 1 is smaller than graph 0):
     customPlot->graph(1)->rescaleAxes(true);

     customPlot->setInteractions(QCP::iRangeDrag | QCP::iRangeZoom | QCP::iSelectPlottables);

     customPlot->replot();
     customPlot->show();

方法2：将低于x轴的遮盖，再绘制一次曲线

    QCustomPlot *plot = new QCustomPlot();
    customPlot->setMinimumSize(600, 400);
    // Generate datasets to plot
    QVector<double> x(1000), y(1000);
    for (int i = 0; i < 1000; ++i) {
        x[i] = i / 10.0;
        y[i] = sin(x[i]) * exp(-x[i] / 10.0) + 0.1 * (rand() / (double)RAND_MAX - 0.5);
    }

    // create and configure the graph
    QCPGraph *graph = customPlot->addGraph();
    graph->setData(x, y);
    graph->setPen(QPen(Qt::black));
    graph->setBrush(QBrush(Qt::black));

    // find the minimum and maximum y values
    double ymin = INFINITY, ymax = -INFINITY;
    for (int i=0; i<y.size(); ++i)
    {
        if (y[i] < ymin)
            ymin = y[i];
        if (y[i] > ymax)
            ymax = y[i];
    }

    // create and configure the rect item to cover the area below the x axis
    QCPItemRect *rect = new QCPItemRect(customPlot);
    rect->setBrush(QBrush(Qt::white));
    
    // set QCPItemRect only display inside of the axis
    rect->setClipToAxisRect(true);
    rect->setPen(QPen(Qt::NoPen));
    rect->topLeft->setCoords(x.first(), ymin);
    rect->bottomRight->setCoords(x.last(), 0);
    QCPGraph *graph1 = customPlot->addGraph();
    graph1->setData(x, y);
    graph1->setPen(QPen(Qt::black));
    graph1->setBrush(Qt::NoBrush);

    // set blank axis lines:
    customPlot->rescaleAxes();
    customPlot->xAxis->grid()->setVisible(false);
    customPlot->yAxis->grid()->setVisible(false);

    // make top right axes clones of bottom left axes:
    customPlot->axisRect()->setupFullAxesBox();
    // set the range and labels for the axes
    customPlot->xAxis->setRange(x.first(), x.last());
    customPlot->xAxis->setLabel("X Axis");
    customPlot->yAxis->setRange(ymin, ymax);
    customPlot->yAxis->setLabel("Y Axis");

    // show the plot
    customPlot->replot();
    customPlot->show();