how to change the intensity of curve colour in Qcustomplot with respect to x-axis?

I have a problem where I have to plot a ray from some source.At source the intensity should be strongest and should decrease with distance which is my xaxis.If I am using blue colour to plot my ray than it should be light blue at origin and should darken with distance.

I have attached a QCpcurve to QCustomplot.

There are two vectors say X and Y which I have to plot

Curve.setpen(blue);
Curve.setdata(X,Y);

Problem is that how to change the colour intensity as distance increases.

Please help

You can set a color gradient to QPen by displaying the look you want.

QPen::QPen(const QBrush &brush, qreal width, Qt::PenStyle style = Qt::SolidLine, Qt::PenCapStyle cap = Qt::SquareCap, Qt::PenJoinStyle join = Qt::BevelJoin)

Constructs a pen with the specified brush, width, pen style, cap style and join style.

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QCustomPlot *customplot = new QCustomPlot;
    customplot->setWindowTitle("Gradient Color");
    customplot->resize(640, 480);
    QCPCurve curve(customplot->xAxis, customplot->yAxis);
    QVector<double> x, y;
    for(int i=0; i < 1000; i++){
        double x_ = qDegreesToRadians(i*1.0);
        x << x_;
        y << qCos(x_)*qExp(-0.2*x_);
    }
    customplot->xAxis->setRange(0, qDegreesToRadians(1000.0));
    customplot->yAxis->setRange(-1, 1);

    QLinearGradient gradient(customplot->rect().topLeft(), customplot->rect().topRight());
    gradient.setColorAt(0.0, QColor::fromRgb(14, 11, 63));
    gradient.setColorAt(1.0, QColor::fromRgb(58, 98, 240));
    QPen pen(gradient, 5);
    curve.setPen(pen);

    curve.setData(x, y);
    customplot->show();

    return a.exec();
}