Vertically aligning multiple plots

[SeanM]

I've got multiple plots that I'm attempting to line up vertically in a QVBoxLayout. Each plot has one curve, and I've got the legend turned on. The problem appears to be that layout space is allocated to the legend first based on the length of the legend items' text, and the canvas gets whatever is left. Since my data has different names, the legend associated with each plot takes up a different amount of horizontal space, which causes the plot canvases to have different widths from each other.

I've tried calling plot->legend()->setFixedWidth(), but that doesn't seem to cause the plot to resize.

[Cah]

I've tried calling plot->legend()->setFixedWidth(), but that doesn't seem to cause the plot to resize.

try....
static_cast<QwtLegend*>(plot->legend())->contentsWidget()->setFixedWidth(...);

contentsWidget() returns the parent (widget) of all legend items

[Uwe]

There are different ways to implement this - one is to use a QGridLayout for plots and legends. Instead of QwtPlot::insertLegend() you would have to do something like this:

QwtLegend *legend = new QwtLegend;
legend->setMaxColumns( 1 );
 
connect( plot, SIGNAL( legendDataChanged( const QVariant &, const QList<QwtLegendData> & ) ),
    legend, SLOT( updateLegend( const QVariant &, const QList<QwtLegendData> & ) ) );

QwtLegend *legend = new QwtLegend;
legend->setMaxColumns( 1 );
         
connect( plot, SIGNAL( legendDataChanged( const QVariant &, const QList<QwtLegendData> & ) ),
    legend, SLOT( updateLegend( const QVariant &, const QList<QwtLegendData> & ) ) );

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Uwe

[SeanM]

Thanks Uwe, that seems to work, mostly! The only issue I'm having now is that some of the legends that have longer strings are getting a horizontal scrollbar showing up, so I think I need to modify the minimum width of all the legends to be the appropriate size for the longest string. The good news is that the strings don't changed for the life of the application.

What's the easiest way to figure out the size I need? I feel like I'm looking in QwtLegend for some function that returns a list of QwtLegendLabels like

QList< QWidget * >  legendWidgets (const QVariant &) const

QList< QWidget * >  legendWidgets (const QVariant &) const

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but I guess I'm not sure what I'm supposed to put in for the QVariant object?

[Uwe]

When you see a scroll bar it is from an internal QScrollArea, where your legend items is inside. In your situation the scroll area seems to be too small to show the labels - changing the attributes of the labels won't help as they have their correct size.

After putting some debug statements into your code: is the width of the legend below its sizeHint ? If yes what happens when assign a different horizontal QSizePolicy ( should be QSizePolicy::MinimumExpanding )?

Uwe

By the way: you could also derive from QwtAbstractLegend and implement your own legend, where you simply put QwtLegendLabels into a vertical layout. ( f.e. the stockchart example shows a completely different type of legend ).

[SeanM]

When you see a scroll bar it is from an internal QScrollArea, where your legend items is inside. In your situation the scroll area seems to be too small to show the labels - changing the attributes of the labels won't help as they have their correct size.

I didn't mean to change the attribute of the label itself, I meant to query the each label to find out what size it wanted to be, then calling setFixedWidth() on each legend with based on the maximum label size.

After putting some debug statements into your code: is the width of the legend below its sizeHint ?

The size hint width is -1 for some reason? Here's how I'm creating the legends:

QwtLegend* legend = new QwtLegend();
legends.append(legend);
legend->setMinimumHeight(0);
legend->setMaxColumns(1);
connect(plot, SIGNAL(legendDataChanged(QVariant,QList<QwtLegendData>)),
            legend, SLOT(updateLegend(QVariant,QList<QwtLegendData>)));
grid->addWidget(legend, i, 1);

QwtLegend* legend = new QwtLegend();
legends.append(legend);
legend->setMinimumHeight(0);
legend->setMaxColumns(1);
connect(plot, SIGNAL(legendDataChanged(QVariant,QList<QwtLegendData>)),
            legend, SLOT(updateLegend(QVariant,QList<QwtLegendData>)));
grid->addWidget(legend, i, 1);

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And then in my mainWidget::resizeEvent() I've got:

foreach (QwtLegend* legend, legends)
{
    qDebug("legend width %d, size hint width %d", legend->width(), legend->sizeHint().width());
}

foreach (QwtLegend* legend, legends)
{
    qDebug("legend width %d, size hint width %d", legend->width(), legend->sizeHint().width());
}

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Which produces:
legend width 437, size hint width -1

If yes what happens when assign a different horizontal QSizePolicy ( should be QSizePolicy::MinimumExpanding )?

But I don't think I want MinimumExpanding for the legends, do I? I've got MinimumExpanding for the QwtPlots in column 0, if I set MinimumExpanding on the QwtLegends the QGridLayout gives 50% width to each column, which isn't what I want. Although I guess could play around with QGridLayout::setColumnStretch(). It seems like I want MinimumExpanding on the plots and Maximum or Preferred on the legends, so the legends get just enough horizontal space to not have a scrollbar, but the plots get the majority of the screen real estate. But putting Maximum on makes it a little too small and then the scrollbars show up.

[SeanM]

I've attached a quick example that shows where I'm struggling. If you look in the MainWindow constructor I've got a

#if true
...
#else
...
#endif

#if true
...
#else
...
#endif

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sequence. When that preprocessor directive is true, the application looks pretty good, all the plots are lined up, and all the legend items are fully displayed, with no scroll bars showing. If you switch the true to a false then the plots get curves loaded in a slot (which is closer to my applications actual behavior) and in that version the longer strings don't fit as well within their respective legends and some scrollbars are visible.

I haven't tried inheriting from QwtAbstractLegend yet, I've been trying too many variations on this theme, but I might give it a shot...

[Uwe]

The size hint width is -1 for some reason?

Probably you checked too early - before the labels have been inserted. Without a valid sizeHint() the layout can't calculate anything useful.
What is the situation, when you resize the main window ( = forcing a recalculation of the layout ) later, after all legend items are inserted.

But I don't think I want MinimumExpanding for the legends, do I?

Of course you want to: it tells the layout not to shrink the legend below its sizeHint(). As you want to have all additional space for the plots you do it with layout->selColumnStretch( 0, 10 ),

Uwe

PS: try to feed your layout in a proper way - using setFixedSize is almost always an indication of doing it the wrong way !