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Marker - QwtPlotMarker

QwtPlotMarker is a plot item for marking specific positions on a plot. It can display horizontal lines, vertical lines, crosshair lines, symbols, or text labels, and is commonly used to annotate key data points, threshold lines, or reference positions.

Key Features

  • Multiple line styles: Horizontal line, vertical line, crosshair, or no line
  • Symbol markers: Display custom symbols at specified positions
  • Text labels: Add text annotations with controllable position and orientation
  • Precise positioning: Located using coordinate values, automatically follows axis transformations

Basic Concepts

Line Style Types

QwtPlotMarker supports four line styles:

Style Enum Value Description
No line NoLine Show symbols and labels only
Horizontal line HLine Horizontal line through the marker point
Vertical line VLine Vertical line through the marker point
Crosshair Cross Combination of horizontal and vertical lines

Marker Composition

A single marker can combine several elements around one coordinate point: a horizontal line, a vertical line, a symbol drawn on the point itself, and a text label anchored nearby. The ASCII sketch below shows how these parts are composed around the (x, y) position:

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         Label text
            ↓
    ────────●────────  ← Horizontal line + symbol
            │
            │          ← Vertical line
            │
    (x, y) Coordinate point

Usage

1. Creating a Basic Marker

The marker position is given in plot data coordinates via setValue() (or setXValue() / setYValue() for a single axis), and the item becomes visible once it is attach()ed to the plot. The example below places a marker at (5, 10):

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#include <QwtPlot>
#include <QwtPlotMarker>

QwtPlot* plot = new QwtPlot();

// Create marker
QwtPlotMarker* marker = new QwtPlotMarker();

// Set marker position (coordinate values)
marker->setXValue(5.0);
marker->setYValue(10.0);
// Or use setValue(double x, double y)
marker->setValue(5.0, 10.0);

// Attach to plot
marker->attach(plot);

plot->replot();

2. Horizontal Line Marker

Horizontal lines are commonly used to mark thresholds, baseline values, or Y-axis reference lines:

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QwtPlotMarker* hLine = new QwtPlotMarker();

// Set line style to horizontal
hLine->setLineStyle(QwtPlotMarker::HLine);

// Set Y coordinate (position of the horizontal line)
hLine->setYValue(50.0);  // Horizontal line at Y=50

// Set line style
hLine->setLinePen(QPen(Qt::red, 2.0, Qt::DashLine));

// Optional: Add label
hLine->setLabel(QwtText("Threshold Line"));
hLine->setLabelAlignment(Qt::AlignRight | Qt::AlignTop);

hLine->attach(plot);

3. Vertical Line Marker

Vertical lines are used to mark time points, event positions, or X-axis reference lines:

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QwtPlotMarker* vLine = new QwtPlotMarker();

// Set line style to vertical
vLine->setLineStyle(QwtPlotMarker::VLine);

// Set X coordinate
vLine->setXValue(100.0);  // Vertical line at X=100

// Set line style
vLine->setLinePen(QPen(Qt::blue, 1.5, Qt::DotLine));

vLine->attach(plot);

4. Crosshair Marker

Crosshair lines display horizontal and vertical intersecting lines at a specific coordinate point:

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QwtPlotMarker* cross = new QwtPlotMarker();

// Set crosshair style
cross->setLineStyle(QwtPlotMarker::Cross);

// Set center position
cross->setValue(50.0, 50.0);

// Set line style
cross->setLinePen(QPen(Qt::green, 1.0, Qt::SolidLine));

cross->attach(plot);

5. Symbol Marker

Display a symbol at a specific position (without lines):

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#include <QwtSymbol>

QwtPlotMarker* symbolMarker = new QwtPlotMarker();

// No lines displayed
symbolMarker->setLineStyle(QwtPlotMarker::NoLine);

// Set position
symbolMarker->setValue(3.0, 8.0);

// Create symbol
QwtSymbol* symbol = new QwtSymbol(
    QwtSymbol::Diamond,
    QBrush(Qt::red),
    QPen(Qt::darkRed, 2),
    QSize(12, 12)
);
symbolMarker->setSymbol(symbol);

symbolMarker->attach(plot);

6. Text Label Configuration

Add text annotations to markers:

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#include <QwtText>

QwtPlotMarker* labelMarker = new QwtPlotMarker();
labelMarker->setValue(10.0, 20.0);
labelMarker->setLineStyle(QwtPlotMarker::Cross);

// Create text label
QwtText label("Key Point (10, 20)");
label.setColor(Qt::black);
label.setFont(QFont("Arial", 10, QFont::Bold));

// Set label
labelMarker->setLabel(label);

// Set label alignment
// Position relative to the marker point
labelMarker->setLabelAlignment(Qt::AlignLeft | Qt::AlignBottom);

// Set label orientation
labelMarker->setLabelOrientation(Qt::Horizontal);  // Horizontal text
// Or labelMarker->setLabelOrientation(Qt::Vertical);  // Vertical text

// Set spacing between label and marker point
labelMarker->setSpacing(5);  // 5 pixel spacing

labelMarker->attach(plot);

7. Full Example - Annotating Data Peaks

This example combines line style, symbol and label in one marker: it scans the curve samples for the maximum, then annotates the peak with a dashed QwtPlotMarker::VLine, a red triangular QwtSymbol and a QwtText label showing the peak value:

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// Find data peak and annotate it
QwtPlotCurve* curve = new QwtPlotCurve("Data");
curve->setSamples(xData, yData, count);
curve->attach(plot);

// Find the maximum value point
double maxVal = yData[0];
int maxIdx = 0;
for (int i = 1; i < count; i++) {
    if (yData[i] > maxVal) {
        maxVal = yData[i];
        maxIdx = i;
    }
}

// Create peak marker
QwtPlotMarker* peakMarker = new QwtPlotMarker();
peakMarker->setValue(xData[maxIdx], yData[maxIdx]);

// Set style
peakMarker->setLineStyle(QwtPlotMarker::VLine);  // Vertical reference line
peakMarker->setLinePen(QPen(Qt::red, 1, Qt::DashLine));

// Set symbol
QwtSymbol* peakSymbol = new QwtSymbol(
    QwtSymbol::Triangle,
    QBrush(Qt::red),
    QPen(Qt::darkRed, 1),
    QSize(10, 10)
);
peakMarker->setSymbol(peakSymbol);

// Set label
QwtText label(QString("Peak: %1").arg(maxVal));
label.setColor(Qt::red);
peakMarker->setLabel(label);
peakMarker->setLabelAlignment(Qt::AlignTop | Qt::AlignHCenter);

peakMarker->attach(plot);

Label Alignment Positions

Label alignment relative to the marker point is controlled by Qt::AlignmentFlag:

Alignment Position Description
Qt::AlignLeft | Qt::AlignTop Upper left
Qt::AlignRight | Qt::AlignTop Upper right
Qt::AlignLeft | Qt::AlignBottom Lower left
Qt::AlignRight | Qt::AlignBottom Lower right
Qt::AlignHCenter | Qt::AlignTop Directly above
Qt::AlignHCenter | Qt::AlignBottom Directly below
Qt::AlignLeft | Qt::AlignVCenter Directly left
Qt::AlignRight | Qt::AlignVCenter Directly right

Core Methods Summary

Method Description
setValue(x, y) Set marker position coordinates
setXValue(x) Set X coordinate only
setYValue(y) Set Y coordinate only
value() Get marker position coordinates
xValue() Get X coordinate
yValue() Get Y coordinate
setLineStyle() Set line style
lineStyle() Get line style
setLinePen() Set line pen
linePen() Get line pen
setSymbol() Set symbol
symbol() Get symbol
setLabel() Set text label
label() Get text label
setLabelAlignment() Set label alignment
setLabelOrientation() Set label orientation
setSpacing() Set label spacing

Tip — Marker Layer Control

Markers should typically be drawn above other plot items. This can be controlled by setting the Z value:

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marker->setZ(100);  // Higher Z value draws on top

Example — Related Examples

  • Curve peak annotation: examples/2D/bode
  • Curve tracker: playground/curvetracker

Screenshot of the curve peak annotation:

Bode Plot