Interval Curve - QwtPlotIntervalCurve¶

QwtPlotIntervalCurve is used to draw interval curves with upper and lower bounds, commonly used to display error ranges, confidence intervals, fluctuation ranges, and similar data. Each data point contains a position value and an interval range.

Key Features¶

Interval representation: Displays upper and lower bounds around the curve
Multiple styles: Supports curve, filled area, symbol, and other display modes
Error bar display: Can draw vertical or horizontal error bar lines
Style customization: Interval boundaries and fill can be configured independently

Basic Concepts¶

Interval Curve Composition¶

The diagram below shows a Tube interval curve: the upper and lower interval bounds form two boundary curves, and the area between them is filled:

     Upper bound
       ───────
      /       \
     / Filled  \    ← Area filled with brush()
    /   area    \
   ────────────
     Lower bound

Boundary curves are stroked with pen(), the area between them filled with brush()

Data Structure¶

Uses QwtIntervalSample to represent interval data, where a position value on one axis is paired with an interval on the other:

class QwtIntervalSample {
public:
    QwtIntervalSample();
    QwtIntervalSample(double value, const QwtInterval& interval);
    QwtIntervalSample(double value, double min, double max);

    double value;           // Position value (curve position)
    QwtInterval interval;   // Interval range [min, max]
};

Curve Styles¶

Style	Enum Value	Description
No curve	`NoCurve`	Don't draw a curve; symbols (e.g. error bars) are still displayed
Tube	`Tube`	Draw the upper/lower bounds as two curves with `pen()` and fill the area between them with `brush()`
User curve	`UserCurve`	Reserved for derived classes that overload `drawSeries()`

Usage¶

1. Basic Interval Curve¶

QwtPlotIntervalCurve consumes QwtIntervalSample data, where each sample pairs a median value with a QwtInterval range, and its CurveStyle (NoCurve / Tube, default Tube) decides how the intervals are rendered. In Tube style setPen() strokes the boundary curves while setBrush() fills the area between them. The example below creates a plot and draws four samples as a translucent band:

#include <QwtPlot>
#include <QwtPlotIntervalCurve>
#include <QwtInterval>

QwtPlot* plot = new QwtPlot();
plot->setTitle("Interval Curve Example");
plot->setCanvasBackground(Qt::white);

// Create interval curve
QwtPlotIntervalCurve* curve = new QwtPlotIntervalCurve("Error Range");

// Set style
curve->setStyle(QwtPlotIntervalCurve::Tube);

// Set the pen for the upper/lower boundary curves
curve->setPen(QPen(Qt::blue, 2));

// Set interval fill style
curve->setBrush(QBrush(QColor(100, 150, 200, 100)));

// Prepare data
QVector<QwtIntervalSample> samples;
samples << QwtIntervalSample(5.0, QwtInterval(4.0, 6.0));
samples << QwtIntervalSample(8.0, QwtInterval(6.5, 9.5));
samples << QwtIntervalSample(10.0, QwtInterval(8.0, 12.0));
samples << QwtIntervalSample(12.0, QwtInterval(10.0, 14.0));

curve->setSamples(samples);
curve->attach(plot);
plot->replot();

2. Error Bar Style¶

CurveStyle has no dedicated error-bar value; error bars are produced by switching the curve style to NoCurve and attaching a QwtIntervalSymbol via setSymbol(): the symbol's Style picks the cap shape (Bar draws a line with end caps), the symbol's own setPen() determines the stroke and setWidth() the cap size. setSymbol() takes ownership of the pointer. The snippet below draws error bars only:

// Draw the per-interval symbols only, no tube
curve->setStyle(QwtPlotIntervalCurve::NoCurve);

// Error bar symbol: line with end caps
QwtIntervalSymbol* symbol = new QwtIntervalSymbol(QwtIntervalSymbol::Bar);
symbol->setPen(QPen(Qt::black, 1));
symbol->setWidth(5);  // Error bar cap width (pixels)

// Ownership of the symbol is transferred to the curve
curve->setSymbol(symbol);

3. Tube Style Configuration¶

In Tube style the upper and lower bounds are stroked with the item's setPen() and the area between them is filled through setBrush() (default Qt::NoBrush, i.e. unfilled). The snippet below configures a red stroke with a translucent pink fill for the tube:

curve->setStyle(QwtPlotIntervalCurve::Tube);

// Boundary curve style (both the upper and the lower bound use this pen)
curve->setPen(QPen(Qt::red, 2));

// Fill style for the area between the boundaries
curve->setBrush(QBrush(QColor(255, 200, 200, 150)));

4. Symbol Configuration¶

QwtIntervalSymbol is constructed with a Style, either Bar for a line with end caps or Box for a plain rectangle, and is tuned via setPen(), setBrush(), and setWidth() (stroke/cap width in pixels) before being assigned to the curve with setSymbol(), which takes ownership of the pointer. The example below creates a blue Bar symbol 8 pixels wide:

#include <QwtIntervalSymbol>

// Create interval symbol
QwtIntervalSymbol* symbol = new QwtIntervalSymbol(QwtIntervalSymbol::Bar);

// Set style
symbol->setPen(QPen(Qt::blue, 1));
symbol->setWidth(8);  // Symbol width (pixels)

curve->setSymbol(symbol);  // Ownership is transferred to the curve

// Symbol styles:
// - NoSymbol: No symbol is drawn
// - Bar: Line with end caps (standard error bar)
// - Box: Plain rectangle using pen() and brush()
// - UserSymbol: Reserved for derived classes of QwtIntervalSymbol

Core Methods Summary¶

Method	Description
`setSamples()`	Set interval data (`QVector<QwtIntervalSample>` or `QwtSeriesData<QwtIntervalSample>*`)
`setStyle()`	Set curve style (`NoCurve` / `Tube`)
`setPen()`	Set the pen stroking the boundary curves (`Tube` style)
`setBrush()`	Set the brush filling the tube area
`setSymbol()`	Set the interval symbol drawn per sample (ownership transferred)
`setPaintAttribute()`	Toggle clipping hints (`ClipPolygons` / `ClipSymbol`)

Tip — Application Scenarios

Experimental data error display

Confidence interval visualization

Fluctuation range display

Prediction interval display

Example — Related Examples

Interval curve demo: See custom examples