Draw x = parable in QCustomplot

Question

If i try to draw an x = form parable I obtain a strange draw. what i see

Searching in internet i didn't find anything, this is the code that I use to plot the graphic:

ui->graphic->addGraph();
    ui->graphic->setInteraction(QCP::iRangeDrag, true);
    ui->graphic->setInteraction(QCP::iRangeZoom, true);

    double temp;

    for (double i = -500; i < 500; i += 0.1) {
        if(parableFormPair.first) {
            xAxisValue.push_back(i);
            temp = aTerm * pow(i, 2) + bTerm * i + cTerm;
            yAxisValue.push_back(temp);
        }
        else {
            yAxisValue.push_back(i);
            temp = aTerm * pow(i, 2) + bTerm * i + cTerm;
            xAxisValue.push_back(temp);
        }
    }

    ui->graphic->graph(0)->setData(xAxisValue, yAxisValue);
    ui->graphic->xAxis->setLabel("x");
    ui->graphic->yAxis->setLabel("y");
    ui->graphic->xAxis->setRange(-15, 15);
    ui->graphic->yAxis->setRange(-15, 15);
    ui->graphic->replot();

ParableFormPait.first tell me if the parable is y = or x = (if is true y =)