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Symbol Styles - QwtSymbol

QwtSymbol is used to define the display symbol style for data points, supporting multiple predefined shapes and custom paths. Symbols can have properties such as size, fill, and border configured, providing rich visual representation for curves and scatter plots.

Main Features

Features

  • ✅ Multiple predefined shapes: Ellipse, rectangle, diamond, triangle, cross, etc.
  • ✅ Custom paths: Supports using QPainterPath to define arbitrary shapes
  • ✅ Size and color control: Configurable symbol dimensions, fill color, border color
  • ✅ Pin point settings: Configurable anchor position of the symbol relative to the data point

Basic Concepts

Symbol Shape List

Shape Enum Value Description
No symbol NoSymbol Do not display symbol
Ellipse Ellipse Circle or ellipse
Rectangle Rect Square or rectangle
Diamond Diamond Diamond
Triangle Triangle Upward-pointing triangle
Down Triangle DTriangle Downward-pointing triangle
Left Triangle LTriangle Left-pointing triangle
Right Triangle RTriangle Right-pointing triangle
Cross Cross Plus-shaped cross
X Cross XCross X-shaped cross
Star Star1 Six-pointed star
Star2 Star2 Five-pointed star
Hexagon Hexagon Regular hexagon
Path Path Custom QPainterPath
Graphic Graphic QwtGraphic object
SVG Svg SVG graphic

Usage

The symbol demo is located at: playground/symbols. Screenshot:

Symbols Demo

1. Creating Basic Symbols

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#include <QwtSymbol>

// Method 1: Convenience constructor
QwtSymbol* symbol = new QwtSymbol(
    QwtSymbol::Ellipse,     // Shape
    QBrush(Qt::red),        // Fill
    QPen(Qt::darkRed, 1),   // Border
    QSize(10, 10)           // Size
);

// Method 2: Step-by-step configuration
QwtSymbol* symbol2 = new QwtSymbol();
symbol2->setStyle(QwtSymbol::Diamond);
symbol2->setBrush(QBrush(Qt::blue));
symbol2->setPen(QPen(Qt::black, 2));
symbol2->setSize(8, 8);

2. Applying Symbols to Curves

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#include <QwtPlotCurve>

QwtPlotCurve* curve = new QwtPlotCurve();

// Set symbol
curve->setSymbol(new QwtSymbol(
    QwtSymbol::Circle,
    QBrush(Qt::yellow),
    QPen(Qt::blue, 1),
    QSize(6, 6)
));

// Symbol will be displayed at each data point position

3. Common Symbol Shape Examples

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// Circle symbol
QwtSymbol* circle = new QwtSymbol(QwtSymbol::Ellipse,
    QBrush(Qt::white), QPen(Qt::blue, 2), QSize(10, 10));

// Rectangle symbol
QwtSymbol* rect = new QwtSymbol(QwtSymbol::Rect,
    QBrush(Qt::green), QPen(Qt::darkGreen), QSize(8, 8));

// Diamond symbol
QwtSymbol* diamond = new QwtSymbol(QwtSymbol::Diamond,
    QBrush(Qt::cyan), QPen(Qt::darkCyan), QSize(12, 12));

// Cross symbol (lines only, no fill)
QwtSymbol* cross = new QwtSymbol(QwtSymbol::Cross,
    Qt::NoBrush, QPen(Qt::red, 2), QSize(10, 10));

// X cross symbol
QwtSymbol* xcross = new QwtSymbol(QwtSymbol::XCross,
    Qt::NoBrush, QPen(Qt::magenta, 2), QSize(12, 12));

// Star symbol
QwtSymbol* star = new QwtSymbol(QwtSymbol::Star1,
    QBrush(Qt::yellow), QPen(Qt::orange), QSize(15, 15));

4. Custom Shapes

Creating custom shapes using QPainterPath:

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QwtSymbol* customSymbol = new QwtSymbol();

// Create custom path (e.g., arrow shape)
QPainterPath path;
path.moveTo(0, -10);     // Top tip
path.lineTo(6, 5);       // Right shoulder
path.lineTo(0, 2);       // Center indent
path.lineTo(-6, 5);      // Left shoulder
path.closeSubpath();

customSymbol->setStyle(QwtSymbol::Path);
customSymbol->setPath(path);
customSymbol->setBrush(QBrush(Qt::red));
customSymbol->setPen(QPen(Qt::darkRed, 1));
customSymbol->setSize(20, 20);

5. Symbol Size Settings

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// Set fixed size
symbol->setSize(QSize(10, 10));  // 10x10 pixels

// Set different width and height (elliptical)
symbol->setSize(15, 8);  // Width 15, height 8

// Dynamic size (based on data value)
// Needs to be implemented in custom drawing

6. Symbol Pin Point

Setting the anchor position of the symbol relative to the data point:

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// Default pin point is at symbol center
symbol->setPinPoint(QPointF(0.5, 0.5));  // Relative position (0.5 is center)

// Set pin point at symbol bottom center
symbol->setPinPoint(QPointF(0.5, 1.0));

// Set pin point at symbol top-left corner
symbol->setPinPoint(QPointF(0.0, 0.0));

// Pin point uses normalized coordinates [0,1]

7. Symbol Fill Styles

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// Simple fill
symbol->setBrush(QBrush(Qt::red));

// Gradient fill
QLinearGradient gradient(0, 0, 0, 1);
gradient.setColorAt(0, Qt::red);
gradient.setColorAt(1, Qt::darkRed);
symbol->setBrush(QBrush(gradient));

// No fill (border only)
symbol->setBrush(Qt::NoBrush);

// Pattern fill
symbol->setBrush(QBrush(Qt::blue, Qt::DiagCrossPattern));

8. Border Styles

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// Solid border
symbol->setPen(QPen(Qt::black, 1, Qt::SolidLine));

// Dashed border
symbol->setPen(QPen(Qt::blue, 2, Qt::DashLine));

// No border
symbol->setPen(Qt::NoPen);

// Rounded border (only valid for rectangles)
// Needs to be implemented via custom Path

Core Method Summary

Method Description
setStyle() Set symbol shape
setSize() Set symbol size
setBrush() Set fill brush
setPen() Set border pen
setPath() Set custom path
setPinPoint() Set pin point position
drawSymbol() Draw symbol

Symbol Usage Recommendations

  • Use small symbols or no symbols when data points are dense
  • Use large symbols to emphasize key data points
  • Use different shapes for different data series for easy distinction
  • Unfilled symbols (Cross, XCross) have better performance

Related Examples

  • Symbol demo: playground/symbols