Problems with QwtPlotScaleItem and switching scale engine

Hi,

we are using a QwtPlotScaleItem to display the axes' ticks inside of the canvas. And in our software the user has the option to switch between a linear and a logarithmic scale for each axis. I compressed our code to a minimum, so the bug is easier to reprocude. First, I derived from QwtPlot and create the QwtPlotScaleItem.

Plot.h

Code:

#include <qwt_plot.h>
 
enum class ScaleType
{
	Linear,
	Logarithmic
};
 
class QwtPlotScaleItem;
 
class Plot : public QwtPlot
{
public:
	Plot();
 
	void setScaleType(ScaleType scaleType);
 
private:
	ScaleType m_scaleType = ScaleType::Linear;
	QwtPlotScaleItem* m_scaleItem = nullptr;
 
	void createScaleItem();
};

Plot.cpp

Code:

#include "Plot.h"
#include <QPen>
#include <qwt_plot_grid.h>
#include <qwt_plot_opengl_canvas.h>
#include <qwt_scale_engine.h>
#include <qwt_scale_widget.h>
#include <qwt_plot_scaleitem.h>
 
Plot::Plot() : QwtPlot()
{
	axisWidget(QwtPlot::xBottom)->scaleDraw()->enableComponent(QwtAbstractScaleDraw::Ticks, false);
	axisWidget(QwtPlot::xBottom)->scaleDraw()->enableComponent(QwtAbstractScaleDraw::Backbone, false);
	createScaleItem();
	m_scaleItem->attach(this);
	setCanvas(new QwtPlotOpenGLCanvas());
	auto* grid = new QwtPlotGrid();
	grid->enableXMin(true);
	grid->enableYMin(true);
	grid->setMajorPen(QPen(Qt::black, 0, Qt::DotLine));
	grid->setMinorPen(QPen(Qt::gray, 0, Qt::DotLine));
	grid->attach(this);
	setAxisAutoScale(QwtPlot::xBottom, true);
}
 
void Plot::setScaleType(ScaleType scaleType)
{
	if (m_scaleType == scaleType)
	{
		return;
	}
	m_scaleType = scaleType;
	auto* scaleEngine = m_scaleType == ScaleType::Linear ?
		static_cast<QwtScaleEngine*>(new QwtLinearScaleEngine) : 
		static_cast<QwtScaleEngine*>(new QwtLogScaleEngine);
 
	const bool isFloatingEnabled = axisScaleEngine(QwtPlot::xBottom)->testAttribute(QwtScaleEngine::Floating);
	setAxisScaleEngine(QwtPlot::xBottom, scaleEngine);
	axisScaleEngine(QwtPlot::xBottom)->setAttribute(QwtScaleEngine::Floating, isFloatingEnabled);
	replot();
}
 
void Plot::createScaleItem()
{
	m_scaleItem = new QwtPlotScaleItem();
	m_scaleItem->setAlignment(QwtScaleDraw::TopScale);
	m_scaleItem->setXAxis(QwtPlot::xBottom);
	m_scaleItem->setScaleDivFromAxis(true);
	m_scaleItem->setFont(axisWidget(QwtPlot::xBottom)->font());
	m_scaleItem->setBorderDistance(0);
	auto* scaleDraw = m_scaleItem->scaleDraw();
	scaleDraw->enableComponent(QwtScaleDraw::Backbone, false);
	scaleDraw->enableComponent(QwtScaleDraw::Labels, false);
	scaleDraw->enableComponent(QwtScaleDraw::Ticks, true);
}

In my dialog I am using the plot and define to buttons to switch between linear and logarithmic scale.
.h

Code:

#include "ui_QwtPlotClient.h"
 
class QwtPlotCurve;
class Plot;
 
class QwtPlotClient : public QDialog
{
	Q_OBJECT
 
public:
	QwtPlotClient(QWidget* parent = nullptr);
 
private:
	Ui::QwtPlotClientClass m_ui;
	Plot* m_plot = nullptr;
	QwtPlotCurve* m_curve = nullptr;
};

.cpp

Code:

#include "QwtPlotClient.h"
#include "Plot.h"
#include <qwt_plot_curve.h>
#include <qwt_symbol.h>
#include <QPushButton>
#include <QSpacerItem>
 
namespace
{
	const QVector<QPointF> linCurvePoints{ {-10, 10}, {20, 0}, {30, 20} , {40, -10} , {50, -5} };
	const QVector<QPointF> logCurvePoints{ {20, 0}, {30, 20} , {40, -10} , {50, -5} };
}
 
QwtPlotClient::QwtPlotClient(QWidget* parent)
	: QDialog(parent),
	m_plot(new Plot()),
	m_curve(new QwtPlotCurve)
{
	m_ui.setupUi(this);
	m_ui.plotFrame->layout()->addWidget(m_plot);
 
	m_curve->setSamples(linCurvePoints);
	m_curve->setPen(Qt::red);
	m_curve->setSymbol(new QwtSymbol(QwtSymbol::Ellipse, QBrush(Qt::red), QPen(Qt::darkRed), QSize(8, 8)));
	m_curve->attach(m_plot);
 
	auto* buttonLayout = new QHBoxLayout;
	m_ui.verticalLayout->addLayout(buttonLayout);
 
	auto button = new QPushButton("Linear");
	connect(button, &QPushButton::clicked, this, [this]()
			  {
				  m_curve->setSamples(linCurvePoints);
				  m_plot->setScaleType(ScaleType::Linear);
			  });
	buttonLayout->addWidget(button);
 
	button = new QPushButton("Log");
	connect(button, &QPushButton::clicked, this, [this]()
			  {
				  m_curve->setSamples(logCurvePoints);
				  m_plot->setScaleType(ScaleType::Logarithmic);
			  });
	buttonLayout->addWidget(button);
 
	buttonLayout->addSpacerItem(new QSpacerItem(10, 10, QSizePolicy::Expanding, QSizePolicy::Fixed));
}

When I start the program, the ticks of the QwtPlotScaleItem are ok - they match perfectly with the grid.
Attachment 13837

When I switch to the log scale, they are still perfectly matched to the grid.
Attachment 13838

But when I now switch back to the linear scale, the (minor) ticks are off the grid.
Attachment 13839

When I resize the plot, everything looks good again. What am I missing or is this a bug?

Important to know: we are using Qt 5.14.0 and I tested this code with Qwt 6.2.0, 6.2.1 and 6.3.0.