qcustomplot 通过鼠标轨迹画矩形框

要通过鼠标轨迹画矩形框，可以结合使用QCustomPlot的鼠标事件和QCPItemRect类。可以按照以下步骤进行操作：

1. 创建一个QCPItemRect对象：

QCPItemRect *rect = new QCPItemRect(customPlot);
rect->setPen(QPen(Qt::red));
rect->setBrush(QBrush(Qt::blue));

2. 实现鼠标事件：

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        rect->topLeft->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        rect->bottomRight->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        rect->setVisible(true);
        ui->customPlot->replot();
    }
}

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
    if (event->buttons() == Qt::LeftButton)
    {
        rect->bottomRight->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        ui->customPlot->replot();
    }
}

void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        rect->setVisible(false);
        ui->customPlot->replot();
    }
}

其中，鼠标按下时记录矩形框左上角的坐标，鼠标移动时更新矩形框右下角的坐标并重新绘制，鼠标释放时隐藏矩形框并重新绘制。

完整的代码示例：

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        rect->topLeft->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        rect->bottomRight->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        rect->setVisible(true);
        ui->customPlot->replot();
    }
}

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
    if (event->buttons() == Qt::LeftButton)
    {
        rect->bottomRight->setCoords(ui->customPlot->xAxis->pixelToCoord(event->pos().x()), ui->customPlot->yAxis->pixelToCoord(event->pos().y()));
        ui->customPlot->replot();
    }
}

void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton)
    {
        rect->setVisible(false);
        ui->customPlot->replot();
    }
}

void MainWindow::setupPlot()
{
    // ...
    rect = new QCPItemRect(ui->customPlot);
    rect->setPen(QPen(Qt::red));
    rect->setBrush(QBrush(Qt::blue));
    rect->setVisible(false);
}