QCustomplot hua yuanquan

    float xRadius = 20;
    float yRadius = xRadius;

    QCPItemEllipse *ellipse = new QCPItemEllipse(ui->customPlot);
    QCPItemTracer *mCenterTracer = new QCPItemTracer(ui->customPlot);
                 mCenterTracer->position->setCoords(ui->customPlot->xAxis->range().upper/2,ui->customPlot->yAxis->range().upper/2);

    mCenterTracer->setStyle(QCPItemTracer::tsNone);
    ellipse->topLeft->setParentAnchor(mCenterTracer->position);
    ellipse->bottomRight->setParentAnchor(mCenterTracer->position);
    ellipse->topLeft->setCoords(-xRadius/2.0, -yRadius/2.0);
    ellipse->bottomRight->setCoords(xRadius/2.0, yRadius/2.0);
    
    QCPItemLine *line = new QCPItemLine(customPlot);
    line->start->setCoords(-0.1, 0.8);
    line->end->setCoords(1.1, 0.2);

    ui->customPlot->replot();