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Scale Engine - QwtScaleEngine

QwtScaleEngine is the calculation engine for axis ticks, responsible for automatically calculating reasonable tick positions and labels based on the data range. Different types of scale engines support different tick strategies.

Main Features

Features

  • ✅ Automatic tick calculation: Automatically calculates major and minor ticks based on data range
  • ✅ Multiple scale types: Supports linear, logarithmic, date, and other scale engines
  • ✅ Tick optimization: Automatically selects aesthetically pleasing tick intervals (e.g., 10, 5, 2)
  • ✅ Boundary handling: Supports tick calculation with included/excluded boundaries

Basic Concepts

Scale Engine Types

Engine Class Description
QwtLinearScaleEngine Linear scale engine (default)
QwtLogScaleEngine Logarithmic scale engine
QwtDateScaleEngine Date/time scale engine

Scale Division Structure

QwtScaleDiv represents the scale division result:

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QwtScaleDiv contains:
- Interval: Data range [min, max]
- MajorTicks: List of major tick positions
- MinorTicks: List of minor tick positions
- MediumTicks: List of medium tick positions

Usage

1. Linear Scale Engine (Default)

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#include <QwtPlot>

QwtPlot* plot = new QwtPlot();

// Uses linear scale engine by default
plot->setAxisAutoScale(QwtAxis::XBottom);

// Set maximum number of major ticks
plot->setAxisMaxMajor(QwtAxis::XBottom, 10);  // Up to 10 major ticks

// Set maximum number of minor ticks
plot->setAxisMaxMinor(QwtAxis::XBottom, 5);   // Up to 5 minor ticks per major tick interval

2. Logarithmic Scale Engine

Used for large-range data or exponential relationship data:

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#include <QwtLogScaleEngine>

// Set logarithmic scale engine
plot->setAxisScaleEngine(QwtAxis::YLeft, new QwtLogScaleEngine());

// Set logarithmic scale range
plot->setAxisScale(QwtAxis::YLeft, 0.001, 1000);  // Spans 6 orders of magnitude

// Logarithmic scale characteristics:
// - Major ticks at 10^n positions (1, 10, 100, 1000...)
// - Minor ticks at intermediate positions (2, 3, 4, 5, 6, 7, 8, 9)

3. Date Scale Engine

Used for time series data:

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#include <QwtDateScaleEngine>
#include <QwtDateScaleDraw>

// Set date scale engine
plot->setAxisScaleEngine(QwtAxis::XBottom, new QwtDateScaleEngine());

// Set date scale drawing
QwtDateScaleDraw* dateDraw = new QwtDateScaleDraw();
dateDraw->setDateFormat(QwtDateScaleDraw::Month, QString("yyyy-MM"));  // Month format
dateDraw->setDateFormat(QwtDateScaleDraw::Day, QString("MM-dd"));      // Day format
plot->setAxisScaleDraw(QwtAxis::XBottom, dateDraw);

// Set time range
QDateTime start = QDateTime::fromString("2024-01-01", "yyyy-MM-dd");
QDateTime end = QDateTime::fromString("2024-12-31", "yyyy-MM-dd");
plot->setAxisScale(QwtAxis::XBottom,
    QwtDate::toDouble(start),
    QwtDate::toDouble(end));

4. Custom Scale Division

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#include <QwtScaleDiv>

// Manually create scale division
QwtScaleDiv scaleDiv;
scaleDiv.setInterval(0, 100);  // Set range

// Set major ticks
QList<double> majorTicks;
majorTicks << 0 << 20 << 40 << 60 << 80 << 100;
scaleDiv.setTicks(QwtScaleDiv::MajorTick, majorTicks);

// Set minor ticks
QList<double> minorTicks;
for (int i = 0; i <= 100; i += 4) {
    if (!majorTicks.contains(i))
        minorTicks << i;
}
scaleDiv.setTicks(QwtScaleDiv::MinorTick, minorTicks);

// Apply scale division
plot->setAxisScaleDiv(QwtAxis::XBottom, scaleDiv);

5. Scale Engine Properties

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// Get scale engine
QwtScaleEngine* engine = plot->axisScaleEngine(QwtAxis::XBottom);

// Set engine properties
engine->setAttribute(QwtScaleEngine::IncludeReference, true);  // Include reference value
engine->setAttribute(QwtScaleEngine::Symmetric, false);        // Asymmetric range
engine->setAttribute(QwtScaleEngine::Floating, false);         // Non-floating endpoints

// Set margins
engine->setMargins(0.05);  // 5% margin

6. Calculating Scale Division

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#include <QwtScaleEngine>

// Manually use engine to calculate ticks
QwtLinearScaleEngine engine;
QwtScaleDiv scaleDiv = engine.divideScale(
    0, 100,        // Data range
    10, 5,         // Max major ticks, max minor ticks
    0, 100         // Reference range (optional)
);

// Apply result
plot->setAxisScaleDiv(QwtAxis::XBottom, scaleDiv);

Scale Engine Property Description

Property Description
IncludeReference Force inclusion of reference value as a tick
Symmetric Force symmetric range
Floating Endpoints not forced to fall on ticks
Inverted Invert tick direction

Date Format Levels

QwtDateScaleDraw supports different time-level formats:

Level Enum Value Description
Millisecond Millisecond level "hh🇲🇲ss.zzz"
Second Second level "hh🇲🇲ss"
Minute Minute level "hh:mm"
Hour Hour level "hh:mm"
Day Day level "MM-dd"
Week Week level "MM-dd"
Month Month level "yyyy-MM"
Year Year level "yyyy"

Core Method Summary

Method Description
setAxisScaleEngine() Set scale engine
setAxisAutoScale() Enable auto scaling
setAxisMaxMajor() Set maximum major ticks
setAxisMaxMinor() Set maximum minor ticks
axisScaleEngine() Get scale engine
axisScaleDiv() Get scale division

Scale Optimization Recommendations

  • Use linear engine for linear data (default)
  • Use logarithmic engine for large-range data (spanning multiple orders of magnitude)
  • Use date engine for time series data
  • Manually set scale division for special requirements

Related Examples

  • Scale engine demo: playground/scaleengine

Screenshot of the scale engine demo:

Scale Engine