Here is a subclass that alows you to drag single points of one graph and also add new points if the mouse hits an empty space (you have to set a margin of how many pixel away from a point a new one should be created):

The Header:

#ifndef EQWIDGET_H
#define EQWIDGET_H
#include "qcustomplot.h"
class EQWidget : public QCustomPlot
{
Q_OBJECT
public:
EQWidget(QWidget* parent);
void mousePressEvent(QMouseEvent *event);
void mouseReleaseEvent(QMouseEvent *event);
void mouseMoveEvent(QMouseEvent *event);
void setDragInfo( int graph = -1, int distance = -1);
signals:
void DataChanged( void );
void EditingFinished(void);
private:
int drag_number = -1;
int dragable_graph_number = -1;
int max_distance_to_add_point = -1;
};
#endif // EQWIDGET_H

The .cpp:

#include "eqwidget.h"
EQWidget::EQWidget(QWidget* parent) : QCustomPlot(parent)
{
}
void EQWidget::setDragInfo(int graph, int distance)
{
dragable_graph_number = graph;
max_distance_to_add_point = distance;
}
void EQWidget::mousePressEvent(QMouseEvent *event)
{
double x,y;
double best_dist = 1E+300;
int best_index = 0;
if( (dragable_graph_number >= 0) && ( dragable_graph_number < this->graphCount()) )
{
QCPGraph* pq_graph = this->graph(dragable_graph_number);
pq_graph->pixelsToCoords(event->localPos(),x,y);
if( pq_graph->data()->size() >= 1 )
{
for( int n=0; n<(pq_graph->data()->size()); n++ )
{
double dist = fabs( (pq_graph->data()->begin()+n)->value - y );
dist += fabs( (pq_graph->data()->begin()+n)->key - x );
if( dist < best_dist )
{
best_dist = dist;
best_index = n;
}
}
if( max_distance_to_add_point > 0 )
{
QPointF q_pos_gui = pq_graph->coordsToPixels( (pq_graph->data()->begin()+best_index)->key, (pq_graph->data()->begin()+best_index)->value );
int dist_px = fabs( event->localPos().x() - q_pos_gui.x()) + fabs( event->localPos().y() - q_pos_gui.y());
if( dist_px/2 > max_distance_to_add_point )
{
pq_graph->addData(x,y);
pq_graph->data().data()->sort();
for( int n=0; n<(pq_graph->data()->size()); n++ )
{
if( (pq_graph->data()->begin()+n)->value == y && (pq_graph->data()->begin()+n)->key == x ) best_index = n;
}
}
}
drag_number = best_index;
}
}
QCustomPlot::mousePressEvent(event);
}
void EQWidget::mouseReleaseEvent(QMouseEvent *event)
{
drag_number = -1;
if( (dragable_graph_number >= 0) && ( dragable_graph_number < this->graphCount()) )
{
this->graph(dragable_graph_number)->data().data()->sort();
}
this->replot();
emit( EditingFinished() );
QCustomPlot::mouseReleaseEvent(event);
}
void EQWidget::mouseMoveEvent(QMouseEvent *event)
{
double x,y;
if( (dragable_graph_number >= 0) && ( dragable_graph_number < this->graphCount()) )
{
QCPGraph* pq_graph = this->graph(dragable_graph_number);
pq_graph->pixelsToCoords(event->localPos(),x,y);
y = round( y*16 ) / 16; //snap to grid
x = round( x*4 ) / 4; //snap to grid
if( drag_number >= 0 )
{
(pq_graph->data()->begin()+drag_number)->value = y;
(pq_graph->data()->begin()+drag_number)->key = x;
emit( DataChanged() );
this->replot();
}
}
}

usage:

ui->eqplot->addGraph();
ui->eqplot->setDragInfo(0, 20); // data of graph(0) is dragable, 20px margin
connect(ui->eqplot,&EQWidget::DataChanged,this,&MainWindow::RebuildResultGraph);

DataChanged(void) and EditingFinished(void) are the signals you can use to react on the user interaction.

you can modify the code to meet you requirments..