Scale displayed incorrectly

[Crowley]

I think I discovered a bug in the way that the scale is displayed.

I have an example program with a bare QwtPlot and a checkbox that sets/clears the attribute QwtScaleEngine::Floating for the x axis. Here is the code:

QtDialogTest::QtDialogTest(QWidget* parent)
	: QDialog(parent)
{
	ui.setupUi(this);
	Populate();
	ui.plot->setAxisAutoScale(QwtPlot::xBottom);
	connect(ui.enableFloatingAttributecheckBox, &QCheckBox::stateChanged, this, &QtDialogTest::EnableFloatingAttribute);
}
 
void QtDialogTest::EnableFloatingAttribute(int state)
{
	bool enable = (state == Qt::Checked);
	ui.plot->axisScaleEngine(QwtPlot::xBottom)->setAttribute(QwtScaleEngine::Floating, enable);
	ui.plot->replot();
}
 
void QtDialogTest::Populate()
{
	QVector<QPointF> samples{ QPointF(1000.4, 1.0), QPointF(1001.4, 1.5), QPointF(1001.7, 2.0), QPointF(1001.9, 1.5), QPointF(1002.05, 1.0) };
	auto curve = new QwtPlotCurve;
	curve->setSamples(samples);
	QBrush brush(QColor::fromRgb(255, 0, 0));
	QPen pen(QColor::fromRgb(128, 0, 0));
	curve->setSymbol(new QwtSymbol(QwtSymbol::Ellipse, brush, pen, QSize(8, 8)));
	curve->setPen(pen);
	curve->attach(ui.plot);
}

QtDialogTest::QtDialogTest(QWidget* parent)
	: QDialog(parent)
{
	ui.setupUi(this);
	Populate();
	ui.plot->setAxisAutoScale(QwtPlot::xBottom);
	connect(ui.enableFloatingAttributecheckBox, &QCheckBox::stateChanged, this, &QtDialogTest::EnableFloatingAttribute);
}

void QtDialogTest::EnableFloatingAttribute(int state)
{
	bool enable = (state == Qt::Checked);
	ui.plot->axisScaleEngine(QwtPlot::xBottom)->setAttribute(QwtScaleEngine::Floating, enable);
	ui.plot->replot();
}

void QtDialogTest::Populate()
{
	QVector<QPointF> samples{ QPointF(1000.4, 1.0), QPointF(1001.4, 1.5), QPointF(1001.7, 2.0), QPointF(1001.9, 1.5), QPointF(1002.05, 1.0) };
	auto curve = new QwtPlotCurve;
	curve->setSamples(samples);
	QBrush brush(QColor::fromRgb(255, 0, 0));
	QPen pen(QColor::fromRgb(128, 0, 0));
	curve->setSymbol(new QwtSymbol(QwtSymbol::Ellipse, brush, pen, QSize(8, 8)));
	curve->setPen(pen);
	curve->attach(ui.plot);
}

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When the plot is displayed, everything seems ok (see attached file ScaleOk.png). However, when I reduce the size of the plot, the scale is not displayed correctly anymore (see attached file ScaleWrong.png). I guess the plot attempts to display the leftmost label of the scale and - correctly - reduced the space between the ticks. But since the leftmost data point has a value of 1000.4, which also is the value of the leftmost minor tick, these two are not displayed correctly to each other anymore.

Can you perhaps provide a short-term bugfix for this?

[Uwe]

From the screen shots alone it is hard to say anything useful. Could you create a small and compilable demo where I can debug the situation ?

Uwe

[Crowley]

Sure, see attachment (QwtPlotTest.zip). It includes a dialog class with a QwtPlot widget. When you run the example, resize the dialog by its right border. When the dialog is small enough, you'll see the problem.

As far as I can tell, the problem is that the scale (the x scale in this example) always displays the leftmost number. However, if the number does not fit anymore between the adjecting (minor) ticks, things start to get wrong. The space between the ticks is reduced, but since the leftmost number must still be displayed, the leftmost minor tick starts to move to the right. That is not wrong per se, but the data point is not moved along with the minor tick. Please note that the leftmost data point in my example is exactly the same value as the leftmost minor tick. So when the tick moves, the data point should move, too.

This error does not show when the scale engine attribute QwtScaleEngine::Floating is not set, but unfortunatly that is no option for us.

[Crowley]

Uwe,
did you find something...?

[Uwe]

It's a problem of a missing updates of the axes border distances not being called from QwtPlot::resizeEvent. Fixed in SVN ( all branches >= 6.1 ).
If you don't want to upgrade the following workaround can be done in application code:

virtual void YourPlot::updateLayout override
{
    QwtPlot::updateLayout();
 
    for ( int axisId = 0; axisId < axisCnt; axisId++ )
    {
        if ( axisEnabled( axisId ) )
        {
            QwtScaleWidget* scaleWidget = axisWidget( axisId );
 
            int startDist, endDist;
            scaleWidget->getBorderDistHint( startDist, endDist );
            scaleWidget->setBorderDist( startDist, endDist );
        }
    }
}

virtual void YourPlot::updateLayout override
{
    QwtPlot::updateLayout();

    for ( int axisId = 0; axisId < axisCnt; axisId++ )
    {
        if ( axisEnabled( axisId ) )
        {
            QwtScaleWidget* scaleWidget = axisWidget( axisId );

            int startDist, endDist;
            scaleWidget->getBorderDistHint( startDist, endDist );
            scaleWidget->setBorderDist( startDist, endDist );
        }
    }
}

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Uwe

[Crowley]

Works perfectly, thanx. :-)