What I want to do is just like this:

/* here is the copy from the example code */

// generate some data:
QVector<double> x(101), y(101); // initialize with entries 0..100
for (int i=0; i<101; ++i)
{
  x[i] = i/50.0 - 1; // x goes from -1 to 1
  y[i] = x[i]*x[i]; // let's plot a quadratic function
}
// create graph and assign data to it:
customPlot->addGraph();
customPlot->graph(0)->setData(x, y);
// give the axes some labels:
customPlot->xAxis->setLabel("x");
customPlot->yAxis->setLabel("y");

/* the function I need but not exist */
customPlot->graph(0)->changeData(0.0, 100.0);  // which will change the point to (0.0, 100.0);

// not bad but not good enough
x[0] = 100.0;
customPlot->graph(0)->setData(x, y);
plot->replot();