Arrow Marker - QwtPlotArrowMarker

QwtPlotArrowMarker is a plot item for drawing arrow annotations on plots. It supports configurable start and end points, head and tail endpoint styles, and line styles. It is commonly used for annotating data regions, indicating directions, or adding explanatory notes.

Key Features

• Two positioning modes: explicit start/end coordinates, or start point + pixel length + angle
• Seven endpoint styles: NoEndpoint, ArrowHead, Circle, Square, Diamond, Triangle, CustomPath
• Independent head/tail configuration: each end can have its own style, size, pen, and brush
• Custom paths: create arbitrary endpoint shapes via QPainterPath
• Pixel-level control: in StartLengthAngle mode, arrow size is preserved during zoom

Basic Concepts

Positioning Modes

Mode	Enum Value	Description
Explicit coordinates	ExplicitPoints	Specify start/end in plot coordinates; arrow scales with zoom
Start + length + angle	StartLengthAngle	Specify start point, pixel length, and angle; size is fixed

Endpoint Style Types

Style	Enum Value	Description
None	NoEndpoint	No endpoint drawn
Arrow head	ArrowHead	V-shaped arrowhead
Circle	Circle	Circular endpoint
Square	Square	Square endpoint
Diamond	Diamond	Diamond-shaped endpoint
Triangle	Triangle	Filled triangle endpoint
Custom	CustomPath	Arbitrary shape via QPainterPath

Arrow Structure

The diagram below shows the anatomy of a QwtPlotArrowMarker: it spans from the start point (the tail) to the end point (the head), and each end can carry its own endpoint style.

  tail                                    head
   ↓                                       ↓
   ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━▶
   │                                       │
 start point                          end point

Usage

1. Basic Arrow (ExplicitPoints Mode)

Create an arrow with explicit start and end coordinates (the most common approach):

#include <QwtPlotArrowMarker>

QwtPlotArrowMarker* arrow = new QwtPlotArrowMarker("Arrow Annotation");

// Set start and end points (plot coordinates)
arrow->setPoints(QPointF(1.0, 0.5), QPointF(3.0, 0.8));

// Set line style
arrow->setLinePen(Qt::darkGreen, 2.0);

// Configure head (arrowhead)
arrow->setHeadStyle(QwtPlotArrowMarker::ArrowHead);
arrow->setHeadSize(12.0);
arrow->setHeadBrush(Qt::green);

// Configure tail
arrow->setTailStyle(QwtPlotArrowMarker::Circle);
arrow->setTailSize(8.0);
arrow->setTailBrush(Qt::yellow);

arrow->attach(plot);

2. Fixed-Length Arrow (StartLengthAngle Mode)

In this mode the arrow length is in pixels and does not change when zooming:

QwtPlotArrowMarker* arrow = new QwtPlotArrowMarker("Fixed Arrow");

// Set start point
arrow->setStartPoint(QPointF(5.0, -0.5));

// Switch to length + angle mode
arrow->setPositionMode(QwtPlotArrowMarker::StartLengthAngle);
arrow->setLength(80.0);    // 80 pixels
arrow->setAngle(135.0);    // 135 degrees

// Dashed line style
arrow->setLinePen(QColor(255, 128, 0), 3.0, Qt::DashLine);

// Diamond head
arrow->setHeadStyle(QwtPlotArrowMarker::Diamond);
arrow->setHeadSize(QSizeF(10.0, 15.0));
arrow->setHeadBrush(QColor(255, 200, 0));
arrow->setHeadPen(QPen(Qt::darkRed, 1.5));

arrow->attach(plot);

3. Custom Endpoint Path

Create arbitrary endpoint shapes using QPainterPath, for example a star:

QwtPlotArrowMarker* arrow = new QwtPlotArrowMarker("Custom Arrow");
arrow->setPoints(QPointF(6.0, 0.0), QPointF(8.0, -0.7));
arrow->setLinePen(Qt::magenta, 2.5);

// Create a star path
QPainterPath starPath;
starPath.moveTo(0, -5);
for (int i = 1; i < 5; ++i) {
    double angle = i * 4 * M_PI / 5;
    starPath.lineTo(5 * sin(angle), -5 * cos(angle));
}
starPath.closeSubpath();

// Custom head
arrow->setHeadStyle(QwtPlotArrowMarker::CustomPath);
arrow->setHeadCustomPath(starPath);
arrow->setHeadSize(15.0);
arrow->setHeadBrush(QColor(255, 105, 180));
arrow->setHeadPen(QPen(Qt::darkMagenta, 1.0));

// Square tail
arrow->setTailStyle(QwtPlotArrowMarker::Square);
arrow->setTailSize(10.0);
arrow->setTailBrush(Qt::cyan);

arrow->attach(plot);

4. Dual Triangle Endpoints

Both ends use triangle endpoints, useful for indicating direction:

QwtPlotArrowMarker* arrow = new QwtPlotArrowMarker("Dual Arrow");
arrow->setPoints(QPointF(9.0, 0.8), QPointF(9.0, 0.2));
arrow->setLinePen(Qt::darkBlue, 2.0);

// Head: blue triangle
arrow->setHeadStyle(QwtPlotArrowMarker::Triangle);
arrow->setHeadSize(12.0);
arrow->setHeadBrush(Qt::blue);

// Tail: red triangle
arrow->setTailStyle(QwtPlotArrowMarker::Triangle);
arrow->setTailSize(12.0);
arrow->setTailBrush(Qt::red);

arrow->attach(plot);

Legend Configuration

Arrow markers can appear in the plot legend. When enabled, the icon draws a horizontal sample line with head and tail endpoints:

arrow->setItemAttribute(QwtPlotItem::Legend, true);

The default legend icon size is 24×12 pixels. Adjust it with setLegendIconSize():

arrow->setLegendIconSize(QSize(32, 16));

API Reference

Method	Description
setPoints(start, end)	Set start and end points
setStartPoint(point)	Set start point
setEndPoint(point)	Set end point
setPositionMode(mode)	Set positioning mode
setLength(length)	Set pixel length (StartLengthAngle mode)
setAngle(angle)	Set angle in degrees (StartLengthAngle mode)
setLinePen(pen)	Set arrow line pen
setHeadStyle(style)	Set head endpoint style
setHeadSize(size)	Set head size in pixels
setHeadBrush(brush)	Set head fill brush
setHeadPen(pen)	Set head outline pen
setHeadCustomPath(path)	Set head custom QPainterPath
setTailStyle(style)	Set tail endpoint style
setTailSize(size)	Set tail size in pixels
setTailBrush(brush)	Set tail fill brush
setTailPen(pen)	Set tail outline pen
setTailCustomPath(path)	Set tail custom QPainterPath

Tip — Usage Notes

• Enable anti-aliasing with setRenderHint(QwtPlotItem::RenderAntialiased, true) for smoother lines.
• In StartLengthAngle mode, length is in pixels: the arrow does not resize when zooming.
• In ExplicitPoints mode, start/end use plot coordinates: the arrow scales with axis changes.

Example — Related Examples

• Arrow annotation demo: examples/2D/sinusplot

Screenshot of the arrow annotation: