QWtPlot: Workaround for insertLegend bug?

[AnnaP]

I have the same problem as darkshu. Actually it is really strange because I have a several plots in a few widgets. All the widgets look ok except one. And I can't understand what is wrong (unusual) with that one.
Does anybody know why the problem can appear and how to fix it?

[Uwe]

The same answer: if you want to look into this, send me a small compilable application demonstrating the problem.

Uwe

[AnnaP]

I found where was a problem but I didn't understand why it was so.

Here is a demostration example:

A Qwt plot with a legend

class SimplePlot : public QwtPlot{
 
public:
 
    SimplePlot ();
    SimplePlot (QWidget * widget);
};
 
SimplePlot::SimplePlot() {
 
    QwtLegend *legend = new QwtLegend(this);
    insertLegend(legend, QwtPlot::RightLegend);
 }
 
SimplePlot::SimplePlot (QWidget * widget) : QwtPlot (widget){
 
    QwtLegend *legend = new QwtLegend(this);
    insertLegend(legend, QwtPlot::RightLegend);
 
}

class SimplePlot : public QwtPlot{

public:

    SimplePlot ();
    SimplePlot (QWidget * widget);
};

SimplePlot::SimplePlot() {

    QwtLegend *legend = new QwtLegend(this);
    insertLegend(legend, QwtPlot::RightLegend);
 }

SimplePlot::SimplePlot (QWidget * widget) : QwtPlot (widget){

    QwtLegend *legend = new QwtLegend(this);
    insertLegend(legend, QwtPlot::RightLegend);

}

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A code with a bug:

QTabWidget  * tabWidget     = new QTabWidget;
    QWidget     * plotWidget    = new QWidget;
    SimplePlot  * plot          = new SimplePlot (plotWidget);  // a problem
    //SimplePlot  * plot          = new SimplePlot;             // no problems
 
    QVBoxLayout * plotLayout = new QVBoxLayout;
    plotLayout->addWidget(plot);
    plotWidget->setLayout(plotLayout);
 
    tabWidget->insertTab(0, new QLabel(), "Test Label");
    tabWidget->insertTab(1, plotWidget, "Plot");
 
    tabWidget->setMinimumSize(400, 400);
    tabWidget->show();

QTabWidget  * tabWidget     = new QTabWidget;
    QWidget     * plotWidget    = new QWidget;
    SimplePlot  * plot          = new SimplePlot (plotWidget);  // a problem
    //SimplePlot  * plot          = new SimplePlot;             // no problems

    QVBoxLayout * plotLayout = new QVBoxLayout;
    plotLayout->addWidget(plot);
    plotWidget->setLayout(plotLayout);

    tabWidget->insertTab(0, new QLabel(), "Test Label");
    tabWidget->insertTab(1, plotWidget, "Plot");

    tabWidget->setMinimumSize(400, 400);
    tabWidget->show();

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P.S. if I firstly insert the "Plot" tab and secondly the "Test Label" tab everything also looks fine (even if I use a constructor with a parent widget)

[Uwe]

When the plot is initially on a hidden tab it gets a polish request in a hidden state and the resize event later before it gets displayed ( what could be considered as a Qt bug ). In such a situation the plot layout gets calculated for the widget default size ( 100x30 ) with an invalid height for the vertical scales. The QwtPlot layout code can't handle this with the result, that can be seen on your screenshot.

To avoid this problem I set a default size of 200x200 in the constructor to have something valid, when the polish request is processed. Note, that you will never see the plot widget in its default size, as its size is always changed by your application layout before it gets visible.

The fix is in SVN ( 5.2 + trunk branches ). If you don't want to update your installed Qwt version you can simply do this in your application code:

QwtPlot *plot = new QwtPlot( ...);
plot->resize( 200, 200 );
...

QwtPlot *plot = new QwtPlot( ...);
plot->resize( 200, 200 );
...

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Uwe