I was using Qcustomplot Version: 1.3.2, which supports data map.
const QCPDataMap *dm = graph->data();
With a map, I can find a closer iterator by calling lowerBound().
QCPDataMap::const_iterator it = dm->lowerBound(x);
But, in Version: 2.0.0-beta, the map does not exist any more.
Is there a way to find a closer iterator, like lowerBound()?