本文从我原先发布在 CSDN 的文章迁移而来，原文链接：https://blog.csdn.net/wang112031/article/details/102887191。

绘制折线图时，有时候数据需要做一些标记，如下图所示效果，用到了 QCPItemTracer 类记录备忘

void Storeplot::PlotSetTag(QCPGraph *Graph, double threshold, QVector<QCPItemTracer *> *TagVector)
{
    QSharedPointer<QCPGraphDataContainer> tmpContainer;
    tmpContainer = Graph->data();
    int low = 0, high = tmpContainer->size();
    double x,y;
    qDebug("start TagVector.size()=%d", TagVector->size());
    for(low = 0; low < high; low++)
    {
        y = (tmpContainer->constBegin() + low)->mainValue();

        if(threshold < y)
        {
            QCPItemTracer *tempTracer = new QCPItemTracer(ui->plot);
            tempTracer->setGraph(Graph);
            tempTracer->setInterpolating(true);
            tempTracer->setStyle(QCPItemTracer::tsCircle);
            tempTracer->setPen(QPen(Qt::red));
            tempTracer->setBrush(Qt::red);
            tempTracer->setSize(7);
            tempTracer->setGraphKey((tmpContainer->constBegin() + low)->mainKey());
            TagVector->append(tempTracer);
            //qDebug("i =%d y =%f %f", low, y, (tmpContainer->constBegin() + low)->mainKey());
        }
}
qDebug("TagVector.size()=%d", TagVector->size());
qDebug("high =%d threshold=%f", high, threshold);
ui->plot->replot();
}
void Storeplot::PlotClearTag(QVector<QCPItemTracer *> *TagVector)
{
    qDebug("PlotClearTag TagVector.size()=%d", TagVector->size());
    for(int i = 0; i < TagVector->size(); i++)
    {
        QCPItemTracer *tempTracer = TagVector->at(i);
        tempTracer->setGraph(0);
        ui->plot->removeItem(tempTracer);
        //delete tempTracer;
    }
TagVector->clear();
qDebug("PlotClearTag TagVector.size()=%d", TagVector->size());
ui->plot->replot();
}