Arrow Marker - QwtPlotArrowMarker¶

QwtPlotArrowMarker is a plot item for drawing arrow annotations on plots. It supports customizable start/end points, head/tail endpoint styles, line styles, and more. 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¶

  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

Tips

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.

Related Examples

Arrow annotation demo: examples/2D/sinusplot

Screenshot of the arrow annotation:

Sinusplot