Curve and horizontal bar chart from same set of samples?

[estan]

Hi,

I'm experimenting with Qwt for my plotting needs. It's working very well, but I have a question: Is it possible to make a curve plot and a horizontal bar plot from the same set of samples?

E.g. like this:

// Create a test curve
    QVector<QPointF> samples;
    for (double y = 0.0; y < 1000.0; y += 10.0) {
        auto x = 0.1 + 0.08 * std::sin(y / 50.0);
        samples.append(QPointF(x, y));
    }
 
    // Create a bar chart
    auto barChart = new QwtPlotBarChart("Test Bar Chart");
    barChart->setSamples(samples);
    barChart->setXAxis(QwtPlot::xTop);
    barChart->setRenderHint(QwtPlotItem::RenderAntialiased);
    //barChart->setOrientation(Qt::Horizontal);
    barChart->attach(this);
 
    // Create a curve
    auto curve = new QwtPlotCurve("Test Curve");
    curve->setSamples(samples);
    curve->setXAxis(QwtPlot::xTop);
    curve->setRenderHint(QwtPlotItem::RenderAntialiased);
    curve->attach(this);

// Create a test curve
    QVector<QPointF> samples;
    for (double y = 0.0; y < 1000.0; y += 10.0) {
        auto x = 0.1 + 0.08 * std::sin(y / 50.0);
        samples.append(QPointF(x, y));
    }

    // Create a bar chart
    auto barChart = new QwtPlotBarChart("Test Bar Chart");
    barChart->setSamples(samples);
    barChart->setXAxis(QwtPlot::xTop);
    barChart->setRenderHint(QwtPlotItem::RenderAntialiased);
    //barChart->setOrientation(Qt::Horizontal);
    barChart->attach(this);

    // Create a curve
    auto curve = new QwtPlotCurve("Test Curve");
    curve->setSamples(samples);
    curve->setXAxis(QwtPlot::xTop);
    curve->setRenderHint(QwtPlotItem::RenderAntialiased);
    curve->attach(this);

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With the above code, I get the result in the attache without_horizontal_alignment.png. If I uncomment the

barChart->setOrientation(Qt::Horizontal)

barChart->setOrientation(Qt::Horizontal)

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line, I get the result in with_horizontal_alignment.png.

If I *also* switch the x / y values in my samples vector, I can get the correct result for the bar chart item (see working.png), but then the curve is wrong.

I'm looking for a way to use the same vector of samples and get the look I want ("vertical" line chart, but "horizontal" bar chart).

Thanks in advance.

without_horizontal_alignment.png:

without_horizontal_alignment.jpg

with_horizontal_alignment.png:

with_horizontal_alignment.jpg

working.png:

working.jpg

[Uwe]

Is it possible to make a curve plot and a horizontal bar plot from the same set of samples?

Have you considered to use QwtPlotCurve::Sticks ?

I'm looking for a way to use the same vector of samples and get the look I want ("vertical" line chart, but "horizontal" bar chart).

All setSamples methods are only convenience ending up in setData. So you could derive from QwtPointSeriesData returning a sample with x/y being swapped.

Uwe

[estan]

Thanks a lot for the quick reply Uwe.

Have you considered to use QwtPlotCurve::Sticks ?

Sorry, my example was not quite complete (should have used setMargin(0) and setSpacing(0) on the bar chart). I actually want the look of a real bar chart, since my data points are measurements taken by a spectrometer over a segment of a rock core, so it makes sense to have actual bars that span over this segment. In the end I also think I want to let the user activate error bars, and I think those looks best on a bar chart.

All setSamples methods are only convenience ending up in setData. So you could derive from QwtPointSeriesData returning a sample with x/y being swapped.

I see, yes in the end I think I'll probably make a custom QwtPointSeriesData which is backed by my real data store (a dataset in a HDF5 file).

It was just initially that I was using a QVector of points because it was easy to generate.

But it's good that you confirm that I'll have to swap x/y in order to get the look I want. I somehow thought I'd be able to get the bars to go horizontally using the same input vector as I used for the curve.

(In my real app, I'm not planning to have the two shown at the same time, but let the user switch between line and bar chart, and it would have been convenient if I could use the exact same input data.)

Thanks for the answers!

[Uwe]

I actually want the look of a real bar chart, since my data points are measurements taken by a spectrometer over a segment of a rock core, so it makes sense to have actual bars that span over this segment.

QwtPlotHistogram might be an option here too. It is different to a bar chart as it displays [x1-x2, y] type of samples.

Uwe

[estan]

QwtPlotHistogram might be an option here too. It is different to a bar chart as it displays [x1-x2, y] type of samples.

Ah, thanks. I'll have a look at that too.