I setup candle plot xaxis like bellow:

class MyAxisTickerText : public QCPAxisTickerText
{
protected:
    virtual QVector<double> createTickVector(double tickStep, const QCPRange &range) Q_DECL_OVERRIDE
    {
        Q_UNUSED(tickStep)
        QVector<double> result;
        if (mTicks.isEmpty())
            return result;

        auto start = mTicks.lowerBound(range.lower);
        auto end = mTicks.upperBound(range.upper);
        if (start != mTicks.constBegin()) --start;
        if (end != mTicks.constEnd()) ++end;

        int count = cleanMantissa(std::distance(start, end) / double(mTickCount + 1e-10));

        auto it = start;
        while (it != end) {
            result.append(it.key());
            int step = count;
            if (step == 0) ++it;
            while (--step >= 0 && it != end)
                ++it;
        }

        return result;
    }
};

    QSharedPointer<QCPAxisTickerText> textTickerx(new MyAxisTickerText); 
    textTickerx->setTickCount(10);

    unsigned i = 0;
    for (unsigned int i = 0; i < stockcandledata.size(); i++)
    {
        ... ...
        other codes...
        ... ...

        std::string current_date = stockcandledata[i]["trade_date"]; // date format is like: "2021-11-24"
        QString current_date_qstr = QString::fromUtf8(current_date.c_str());
        textTickerx->addTick(i, current_date_qstr);

    }

    ... ...
    other codes...
    ... ...

    customPlot->xAxis->setTicker(textTickerx); 


now I want to draw a vertical line on date: "2021-11-24"

QCPItemLine *vline = new QCPItemLine(customPlot);
vline->setPen(QColor(144, 11, 219));
vline->start->setCoords( ??? );
vline->end->setCoords( ??? );
vline->setHead(QCPLineEnding::esSpikeArrow);


Please help me how to set the coords of the line?