How to make curves with empty intervals in them?

[alex_sh]

Hello,

I could not find anything related in Qwt docs so I'm posting here. I'm using Qwt6 SVN and I need to draw a curve (actually, a straight horizontal line), but it must not be continuous. I'm attaching a screenshot to show what I mean. The green line runs under the red one, and the red one consists of multiple pieces.
I thought about creating multiple curves for the red one, but I need exactly one legend item for it. Preferably, the settings (color, thickness, etc...) applied to the curve should affect all its pieces.

Any idea how I may accomplish that?

Thanks a lot in advance!

plot_marked.png

[Astronomy]

Hi,
you mean

// Insert markers
 
    //  ...a horizontal line at y = 0...
    QwtPlotMarker *mY = new QwtPlotMarker();
    mY->setLabel(QString::fromLatin1("y = 0"));
    mY->setLabelAlignment(Qt::AlignRight|Qt::AlignTop);
    mY->setLineStyle(QwtPlotMarker::HLine);
    mY->setYValue(0.0);
    mY->attach(this);

// Insert markers
    
    //  ...a horizontal line at y = 0...
    QwtPlotMarker *mY = new QwtPlotMarker();
    mY->setLabel(QString::fromLatin1("y = 0"));
    mY->setLabelAlignment(Qt::AlignRight|Qt::AlignTop);
    mY->setLineStyle(QwtPlotMarker::HLine);
    mY->setYValue(0.0);
    mY->attach(this);

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wouldn't work for you?


Maybe you cann ad a simple constant function:
y = f(x) like y=1 | x[1,2]
e.g.:

// Insert new curves
    QwtPlotCurve *pConstantFunction = new QwtPlotCurve("y = f(1)");
#if QT_VERSION >= 0x040000
    pConstantFunction->setRenderHint(QwtPlotItem::RenderAntialiased);
#endif
    pConstantFunction->setPen(QPen(Qt::red));
    pConstantFunction->setData(....)           //your const f()  or an array of f(..)?
    pConstantFunction->attach(this);

// Insert new curves
    QwtPlotCurve *pConstantFunction = new QwtPlotCurve("y = f(1)");
#if QT_VERSION >= 0x040000
    pConstantFunction->setRenderHint(QwtPlotItem::RenderAntialiased);
#endif
    pConstantFunction->setPen(QPen(Qt::red));
    pConstantFunction->setData(....)           //your const f()  or an array of f(..)?
    pConstantFunction->attach(this);

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or an array of functions?
y = f(x); y=1 | x[1,2]
y = f(x); y=1 | x[3,4]
y = f(x); y=1 | x[7,10]

that will simulate your seperated line?

greetz AP.

[alex_sh]

Astronomy, thanks for your answer.

Hi,
you mean

// Insert markers
 
    //  ...a horizontal line at y = 0...
    QwtPlotMarker *mY = new QwtPlotMarker();
    mY->setLabel(QString::fromLatin1("y = 0"));
    mY->setLabelAlignment(Qt::AlignRight|Qt::AlignTop);
    mY->setLineStyle(QwtPlotMarker::HLine);
    mY->setYValue(0.0);
    mY->attach(this);

// Insert markers
    
    //  ...a horizontal line at y = 0...
    QwtPlotMarker *mY = new QwtPlotMarker();
    mY->setLabel(QString::fromLatin1("y = 0"));
    mY->setLabelAlignment(Qt::AlignRight|Qt::AlignTop);
    mY->setLineStyle(QwtPlotMarker::HLine);
    mY->setYValue(0.0);
    mY->attach(this);

To copy to clipboard, switch view to plain text mode 

wouldn't work for you?

If I understand it correctly, this would not create a legend item which I need (or create multiple items? I'm not sure). A possible workaround would be to add another "dummy" curve with nothing (or a single point) to display, just to get the legend item. I would also need to mirror the curve style changes to the marker (I have a dialog that controls curve appearance). I was hoping for something a bit more straightforward.

Maybe you cann ad a simple constant function:
y = f(x) like y=1 | x[1,2]
e.g.:

// Insert new curves
    QwtPlotCurve *pConstantFunction = new QwtPlotCurve("y = f(1)");
#if QT_VERSION >= 0x040000
    pConstantFunction->setRenderHint(QwtPlotItem::RenderAntialiased);
#endif
    pConstantFunction->setPen(QPen(Qt::red));
    pConstantFunction->setData(....)           //your const f()  or an array of f(..)?
    pConstantFunction->attach(this);

// Insert new curves
    QwtPlotCurve *pConstantFunction = new QwtPlotCurve("y = f(1)");
#if QT_VERSION >= 0x040000
    pConstantFunction->setRenderHint(QwtPlotItem::RenderAntialiased);
#endif
    pConstantFunction->setPen(QPen(Qt::red));
    pConstantFunction->setData(....)           //your const f()  or an array of f(..)?
    pConstantFunction->attach(this);

To copy to clipboard, switch view to plain text mode 

or an array of functions?
y = f(x) y=f(1) | x[1,2]
y = f(x) y=f(1) | x[3,4]
y = f(x) y=f(1) | x[7,10]

that will simulate your seperated line?

You mean just add one curve for each line? That would create multiple legend items (I need only one), and I would have to synchronize the curve appearance settings between them. I'll probably go with this one (once I find out how to hide the curve legend items), unless there is some simpler solution out there.

Thanks

[Astronomy]

Yes the first is just a horizontal line.

The Array of constant functions will produce this:

"_________ _________ ______________"
f1(1). .......f2(1) .........f3(1)

three or more f(..) at the same y-position, but with different x-points will possibly work..

greetz AP.

[Uwe]

1) If you want to display a curve with gaps overload QwtPlotCurve::drawXY ( don't use markers ).
2) By curve->setItemAttribute(QwtPlotItem::Legend, ...) you can control if a curve ( or any other QwtPlotItem ) should have an entry on the legend.

Uwe

[alex_sh]

Thank you both.
I solved this by creating multiple curves and disabling the legend items (using setItemAttribute()) on all of them except the first one.
Note that I had to disable the legend item _before_ the attach() call, or it would still allocate the space in the legend, but draw nothing there.

P.S. There's no QwtPlotCurve::drawXY() in Qwt6, or am I mistaken?

[Galak]

Hi,
I have resolved this problem to using nan values, see the attached files
qwt_plot_piecewise_curve.h
qwt_plot_piecewise_curve.cpp