Tick drawing artifact when setting all plot margins to 0

[estan]

I'm working with the left and top axes enabled. The look I'm getting is:

How can I make the Y axis positioned at X = 0, and the X axis at Y = 0, so that they meet precisely at the origin? E.g. I'd like the axes to look roughly like this:

[estan]

I've tried getting the scale draw of the axes and move()ing it to 0,0, but it seems to have no effect.

[estan]

Sorry, the image above is slightly misrepresentative as I haven't flipped my Y axis yet. It was meant to show the Y axis going from 0 at the top to 1000 at the bottom (how do I do that btw?). My general question is how to position the Y axis at a precise X value, and the X axis at a precise Y value.

[estan]

It seems plotLayout().setAlignCanvasToScales(True) will get me halfway there: There's no longer any space between my curve and the end of the canvas, but there's still a gap between the canvas and the axes backbones.

I tried getting rid of this last space with plotLayout().setSpacing(0), but looks like it had no effect.

[estan]

Alright, this seems to do the trick:

        self.plotLayout().setCanvasMargin(0)
        self.axisWidget(QwtPlot.yLeft).setMargin(0)
        self.axisWidget(QwtPlot.xTop).setMargin(0)

Result:

But as you can see there's some painting artifact at the first major mark of the X axis. Any idea what's going on here?

[PierreRaybaut]

This is apparently a rounding error of some kind, within the code handling the plot layout (here and/or there, or maybe also here).

The render result when setting all margins to 0 is not the same in PythonQwt as in Qwt original C++ library, so there maybe something to dig in the code parts cited above.

This might take some time but it's not hopeless as it works in the original C++ code.

[PierreRaybaut]

As a note for myself:

• This bug is probably related to a rounding error in QwtPlotLayout
• This bug is not happening with PyQwt5 (see screenshots below), so maybe it's working with the original Qwt 6 C++ code and something was lost when porting the code in Python.
• However, let's remember that PythonQwt is based on Qwt 6 whereas PyQwt is based on Qwt 5: in Qwt v6, all widget coordinates are using floating point values (QPointF, QRectF, ...) instead of integer values like version 5 (QPoint, QRect, ...).

Here are two screenshots produced by the newly introduced "plot without margins" test:

PyQwt	PythonQwt
Qwt 5	Qwt 6

[estan]

Thanks a lot for looking into this. I made a small Qwt6 C++ test:

#include <cmath>

#include <QApplication>
#include <QFrame>
#include <QWidget>

#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_layout.h>
#include <qwt_point_data.h>
#include <qwt_scale_widget.h>


class FunctionData : public QwtSyntheticPointData
{
public:
    FunctionData(double (*y)(double)) : QwtSyntheticPointData(500), d_y(y) { }

    virtual double y(double x) const {
        return d_y(x);
    }

private:
    double (*d_y)(double);
};


class TestPlot : public QwtPlot
{
    Q_OBJECT
public:
    TestPlot(QWidget *parent = 0) : QwtPlot(parent) {
        setWindowTitle("Qwt");
        enableAxis(QwtPlot::xTop, true);
        enableAxis(QwtPlot::yRight, true);

        setAxisScale(QwtPlot::xTop, 0, 10);
        setAxisScale(QwtPlot::xBottom, 0, 10);
        setAxisScale(QwtPlot::yRight, -1, 1);
        setAxisScale(QwtPlot::yLeft, -1, 1);

        QwtPlotCurve *curve1 = new QwtPlotCurve("Test Curve 1");
        curve1->setData(new FunctionData(::sin));
        curve1->attach(this);

        qobject_cast<QFrame*>(canvas())->setFrameStyle(QFrame::NoFrame);
        plotLayout()->setCanvasMargin(0);
        axisWidget(QwtPlot::yLeft)->setMargin(0);
        axisWidget(QwtPlot::xTop)->setMargin(0);
        axisWidget(QwtPlot::yRight)->setMargin(0);
        axisWidget(QwtPlot::xBottom)->setMargin(0);
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    TestPlot plot;
    plot.resize(800, 600);
    plot.show();

    return app.exec();
}

#include "moc_qwttest.cpp"

TEMPLATE = app
CONFIG += qwt
TARGET = 
DEPENDPATH += .
INCLUDEPATH += .

# Input
SOURCES += qwttest.cpp
HEADERS += qwttest.cpp

Result:

[estan]

I can also note: At the moment I'm just doing self.plotLayout().setAlignCanvasToScales(True) and setting the margin of the scale widgets to 0, not setting the canvas margin to 0 (to avoid the artifact on the tick mark). This gets me almost the look I want. But the curves are still being "cut off" when touching the axis (as seen in both my example and yours), so would love to have this fixed.

[estan]

@PierreRaybaut: I made a small find:

From qwt_scale_widget.cpp, in QwtScaleWidget::layoutScale:

const QRectF r = contentsRect();

So it will work with a QRectF in the subsequent calculations, but in the corresponding PythonQwt code:

r = self.contentsRect()

So it will work with a QRect.

Could the problem be somewhere around here?

[estan]

I gave it a try, changing it to r = QRectF(self.contentsRect()), but no cigar, not that easy :)

[PierreRaybaut]

Yesterday, I took the time to compare qwt_plot_layout.cpp and plot_layout.py... no luck: everything seems fine.

[PierreRaybaut]

Still investigating, from time to time... but it's like finding a needle in a haystack!

[estan]

Yea, I can imagine :) Wish I had more time to help at the moment. It's no hurry.

I guess it's a matter of going through all related code looking for variables/return values/parameters that should be float or should be int and add assertions of their type.

[JAE-UH]

I suspect the bug is around here, somewhere.

It specifically messes with the yLeft axis and alignCanvasToScales.

I just can't find it yet.

[PierreRaybaut]

4 years later... nailed it ;-)

[estan]

👌