Panning Tool

Note — Panning in Qwt 7

QwtPlotPanner has been refactored for real-time interaction. Panning now moves the live plot content instead of the cached bitmap used by the original QwtPlotPanner.

The Qwt6 version of QwtPlotPanner was based on a cache-based panning mechanism, which had two main issues:

• During panning, a cached bitmap was shown instead of the current plot content
• Data changes were not visible during the pan, so interactive exploration was limited

The Qwt6 QwtPlotPanner reduced plot refreshes by caching, but the cached bitmap made panning feel static. Qwt7 therefore refactored QwtPlotPanner so the plot updates while the mouse moves.

Refactored QwtPlotPanner Improvements

The new QwtPlotPanner uses the QwtPicker state machine for panning and zooming, and updates the plot while the mouse moves:

The animation below shows panning in progress:

The new QwtPlotPanner supports these axis scales:

• Linear Scale
• Logarithmic Scale
• DateTime Scale
• Multiple axes

Warning — Using the Original Cache-Based Picker

The original cache-based panner is available as QwtPlotCachePanner.

Class Changes	Property	Base Class
QwtPanner -> QwtCachePanner	Renamed	QWidget
Original QwtPlotPanner -> QwtPlotCachePanner	Renamed	QwtCachePanner
Original QwtPolarPanner -> QwtPolarCachePanner	Renamed	QwtCachePanner
QwtPlotPanner	Refactored	QwtPicker

The Qwt6 QwtPlotPanner was refactored, but its interface is unchanged, so existing code needs no modification.

Basic Usage

The listing below attaches a QwtPlotPanner to the plot canvas, binds panning to the middle mouse button, and connects the panned signal to report each drag offset.

#include "qwt_plot_panner.h"
#include "qwt_plot.h"
#include "qwt_plot_curve.h"

// Create plot and curve
QwtPlot* plot = new QwtPlot;
QwtPlotCurve* curve = new QwtPlotCurve("Sine Wave");
curve->attach(plot);

// Create real-time panner
QwtPlotPanner* panner = new QwtPlotPanner(plot->canvas());
// Configure mouse button
panner->setMouseButton(Qt::MiddleButton);  // Middle button panning
// panner->setMouseButton(Qt::LeftButton); // Or left button panning

// Enable picker
panner->setEnabled(true);

// Connect signal (optional)
connect(panner, &QwtPlotPanner::panned,
        [](int dx, int dy) {
            qDebug() << "Canvas panned by:" << dx << dy;
        });

You can set the panning direction to restrict movement to one direction:

// 1. Set panning direction
panner->setOrientations(Qt::Horizontal | Qt::Vertical); // Default: horizontal and vertical
// panner->setOrientations(Qt::Horizontal);            // Horizontal only
// panner->setOrientations(Qt::Vertical);              // Vertical only

Note — Axis binding

QwtPlotPanner differs from QwtPlotPicker in that it does not need to bind to axes, because the movement applies to the entire canvas. All axes move with the canvas.