qwtplot and mouse tracking

I have an application with a qwtplot, grid and qwtcurve or histogram_item.h (I think this is irrelevant). I want to show the user the axis values in the plot that correspond to where the cursor is. So I did it like this:

First I declare an eventfilter from qwtplot to a class that I have, and I activate mousetracking:

m_plot->setMouseTracking(true);
m_plot->canvas()->setMouseTracking(true);
m_plot->installEventFilter(this);

in the eventFilter I do this (of course after detecting mouse move event):

statusbar->showMessage( QString("X: ") +
QString::number((int) (0.5 + m_plot->invTransform(QwtPlot::xBottom, mouseEvent->pos().x()))) +
QString(" Y: ") +
QString::number((int) (0.5 + m_plot->invTransform(QwtPlot::yLeft, mouseEvent->pos().y()))));

so basically, I receive mouse position, and I use invTransform method to convert it from pixel position to the values in the plot.

The axis are defined like this:

m_plot->setAxisScale(QwtPlot::yLeft, 0.0, max);
m_plot->setAxisScale(QwtPlot::xBottom, 0.0, numValues);

And now my problem. I have many plots (histograms, curves, etc.) and in all of them, I have an error of measure in the X value, this error depends on the position inside the parent (widget or frame) and/or size of the qwtplot. I don't know if the error is already in the pixels or if it is after invTransform. I can correct the difference if I don't allow to resize the plot, but I want to know where this error comes from.

If you need more code or information, please ask for it, and thanks for your help.

You need to filter the mouse events from the plot canvas not from the plot widget itsself.
Nevertheless have a look at QwtPlotPicker before you continue.

Uwe

Thanks for your answer. I've been busy lately so I haven't tried it. I'll post here if it worked when I have time to work on it.

It worked filtering from canvas(). I won't use qwtplotpicker as the current code works, but I will use it the next time. Thanks again.

Is it possible to use a QwtPlotPicker to show something in the statusbar? I have a Spectrogram and want to show the z-value to the current coordinates (and other things, but that is not so important at the moment). I tried to use "moved"-signal, but that works only if the mouse is clicked (as in "bode"-example).

How can I change that?

got it :) I had to write my own subclass of QwtPlotPicker and overriding the "trackerText" method:

Code:

---

class MyPicker : public QwtPlotPicker
    {
        Q_OBJECT
 
    public:
        MyPicker(int xAxis, int yAxis, int selectionFlags, RubberBand rubberBand, DisplayMode trackerMode, QwtPlotCanvas* canvas);
 
    signals:
        void mouseMoved(const QPoint& pos) const;    
 
    protected:
            virtual QwtText trackerText (const QwtDoublePoint & pos) const;
 
    };

---

Code:

---

MyPicker::MyPicker(int xAxis, int yAxis, int selectionFlags, RubberBand rubberBand, DisplayMode trackerMode, QwtPlotCanvas* canvas)
            : QwtPlotPicker(xAxis, yAxis, selectionFlags, rubberBand, trackerMode, canvas)
        {}
 
QwtText MyPicker::trackerText (const QwtDoublePoint & pos) const
    {
        const QPoint point = pos.toPoint();
        emit mouseMoved(point);
        return QwtText(QString::number(point.x()) + ", " + QString::number(point.y()));
    }

---

now I can react to the "mouseMoved"-signal :)

I'm not sure if this is related to QwtPlotPicker or general to Qt...

as written above, I want to react to the mouseMoved-signal. Currently, I write text into the statusBar. But I want to do this only as long as the mouse is on my plot. When the user moves the mouse outside the canvas, I don't want to do anything. Is there a possibility to check if the cursor is on the canvas? Or the get a signal when the cursor has left the canvas?