Qwt QwtScaleDraw custom coordinate axis

1. Overview

QwtScaleDraw is a class in the Qt drawing library Qwt, used to draw axis ticks and tick labels. It provides methods and properties to set the style, layout, and alignment of tick marks and labels.

The following is the class inheritance relationship:

2. Commonly used methods

Tag related methods:

• setLabelRotation(double angle): Set the label rotation angle.
• setLabelAlignment(Alignment alignment): Set label alignment.

Tick mark related settings:

• void setTickLength (QwtScaleDiv::TickType, double length): Set the tick length

Custom label, override label method

• virtual QwtText label (double) const

3.Example

Customize the coordinate axis of the x-axis.

#ifndef BARCHARTSINGLEWIDGET_H
#define BARCHARTSINGLEWIDGET_H

#include <QWidget>

namespace Ui {
class BarChartSingleWidget;
}

class BarChartSingleWidget : public QWidget
{
    Q_OBJECT

public:
    explicit BarChartSingleWidget(QWidget *parent = 0);
    ~BarChartSingleWidget();

private:
    Ui::BarChartSingleWidget *ui;

    QStringList m_distros;
};

#endif // BARCHARTSINGLEWIDGET_H



#include "BarChartSingleWidget.h"
#include "ui_BarChartSingleWidget.h"
#include "qwt_plot.h"
#include "qwt_plot_curve.h"
#include "qwt_text.h"
#include "qwt_legend.h"
#include "qwt_symbol.h"
#include "qwt_plot_marker.h"
#include "qwt_plot_grid.h"
#include "qwt_scale_div.h"
#include "qwt_plot_canvas.h"
#include "qwt_plot_legenditem.h"
#include "qwt_math.h"
#include "qwt_plot_layout.h"
#include "qwt_plot_barchart.h"
#include "qwt_scale_draw.h"
#include "qwt_column_symbol.h"
#include "qwt_plot_renderer.h"

//custom axis
class ScaleDraw : public QwtScaleDraw
{
  public:
    ScaleDraw( Qt::Orientation orientation, const QStringList & labels )
        : m_labels( labels )
    {
        //Set tick length
        setTickLength( QwtScaleDiv::MinorTick, 0 );
        setTickLength( QwtScaleDiv::MediumTick, 0 );
        setTickLength( QwtScaleDiv::MajorTick, 2 );

        enableComponent( QwtScaleDraw::Backbone, false );

        //Set direction
        if ( orientation == Qt::Vertical )
        {
            setLabelRotation( -60.0 );
        }
        else
        {
            setLabelRotation( -20.0 );
        }

        //Set label alignment
        setLabelAlignment( Qt::AlignLeft | Qt::AlignVCenter );
    }

    //Rewrite label method
    virtual QwtText label(double value) const QWT_OVERRIDE
    {
        QwtText lbl;

        const int index = qRound( value );
        if ( index >= 0 & amp; & amp; index < m_labels.size() )
        {
            lbl = m_labels[index];
        }

        return lbl;
    }

  private:
    const QStringList m_labels;
};

//Customize the ChartItem class and implement the specialSymbol and barTitle methods
class ChartItem : public QwtPlotBarChart
{
  public:
    ChartItem()
        : QwtPlotBarChart( "Page Hits" )
    {
        setLegendMode( QwtPlotBarChart::LegendBarTitles );
        setLegendIconSize( QSize( 10, 14 ) );
        setLayoutPolicy( AutoAdjustSamples );
        setLayoutHint( 4.0 ); // minimum width for a single bar

        setSpacing( 10 ); // spacing between bars
    }

    void addDistro( const QString & amp; distro, const QColor & amp; color )
    {
        m_colors + = color;
        m_distros + = distro;
        itemChanged();
    }

    virtual QwtColumnSymbol* specialSymbol(
        int index, const QPointF & amp; ) const QWT_OVERRIDE
    {
        // we want to have individual colors for each bar
        //Create a new mark
        QwtColumnSymbol* symbol = new QwtColumnSymbol( QwtColumnSymbol::Box );
        symbol->setLineWidth(2); //Set line width
        symbol->setFrameStyle( QwtColumnSymbol::Raised );//Set the border style

        QColor c( Qt::white );
        if ( index >= 0 & amp; & amp; index < m_colors.size() )
            c = m_colors[index];

        //Set color
        symbol->setPalette( c );

        return symbol;
    }

    //Set the title of bar
    virtual QwtText barTitle( int sampleIndex ) const QWT_OVERRIDE
    {
        QwtText title;
        if ( sampleIndex >= 0 & amp; & amp; sampleIndex < m_distros.size() )
            title = m_distros[ sampleIndex ];

        return title;
    }

  private:
    QList<QColor> m_colors; //The color of each bar
    QList<QString> m_distros; //The title of each bar
};

static QwtPlot *g_plot = nullptr;

BarChartSingleWidget::BarChartSingleWidget(QWidget *parent):
    QWidget(parent),
    ui(new Ui::BarChartSingleWidget)
{
    ui->setupUi(this);

    const struct
    {
        const char* distro;
        const int hits;
        QColor color;

    } pageHits[] =
    {
        { "First Grade", 1114, QColor( "DodgerBlue" ) },
        { "Grade 2", 1373, QColor( "#d70751" ) },
        { "Grade 3", 1638, QColor( "SteelBlue" ) },
        { "Grade 4", 1395, QColor( "Indigo" ) },
        { "Fifth Grade", 3874, QColor( 183, 255, 183 ) },
        { "Sixth grade", 1532, QColor( 115, 186, 37 ) },
        { "Grade 7", 1059, QColor( "LightSkyBlue" ) },
        { "Eighth Grade", 2391, QColor( "FireBrick" ) }
    };

    //Set plot background color
    g_plot = new QwtPlot(QwtText("XX school student population statistics"), this);
    g_plot->setAutoFillBackground( true );
    g_plot->setPalette( QColor( "Linen" ) );

    //Set the canvas
    QwtPlotCanvas* canvas = new QwtPlotCanvas();
    canvas->setLineWidth(2);
    canvas->setFrameStyle( QFrame::Box | QFrame::Sunken );
    canvas->setBorderRadius(10);

    //Set the background color of the canvas
    QPalette canvasPalette( QColor( "Plum" ) );
    canvasPalette.setColor( QPalette::WindowText, QColor( "Indigo" ) );
    canvas->setPalette(canvasPalette);

    g_plot->setCanvas(canvas);

    //Create a histogram
    ChartItem* chartItem = new ChartItem();

    //Set bar chart data
    QVector< double > samples;

    for ( uint i = 0; i < sizeof( pageHits ) / sizeof( pageHits[ 0 ] ); i + + )
    {
        m_distros + = pageHits[ i ].distro;
        samples + = pageHits[ i ].hits;

        chartItem->addDistro( pageHits[ i ].distro, pageHits[ i ].color );
    }

    chartItem->setSamples( samples );
    chartItem->attach( g_plot );

    //Set the coordinate axis
    //Set custom axis
    g_plot->setAxisTitle( QwtAxis::XBottom, "grade" );
    g_plot->setAxisMaxMinor( QwtAxis::XBottom, 3 );
    g_plot->setAxisScaleDraw( QwtAxis::XBottom, new ScaleDraw( Qt::Vertical, m_distros ) );

    g_plot->setAxisTitle( QwtAxis::YLeft, "number of people" );
    g_plot->setAxisMaxMinor( QwtAxis::YLeft, 3 );

    //Set custom axis
    QwtScaleDraw* scaleDraw = new QwtScaleDraw();
    scaleDraw->setTickLength( QwtScaleDiv::MediumTick, 4 );
    g_plot->setAxisScaleDraw( QwtAxis::YLeft, scaleDraw );

    g_plot->plotLayout()->setCanvasMargin( 0 );

    //Insert legend
    g_plot->insertLegend( new QwtLegend() );

    g_plot->replot();

    //Display drawing object
    ui->verticalLayout->addWidget(g_plot);
}

BarChartSingleWidget::~BarChartSingleWidget()
{
    delete ui;
}

4. Related references

Detailed explanation of Qwt QwtLegend and QwtPlotLegendItem legend classes-CSDN Blog

Detailed explanation of Qwt QwtPlot class-CSDN Blog

Qwt QwtPlotBarChart custom bar chart-CSDN Blog