JKQtPlotter/examples/ui_bind_scrollbar/README.md

Example (JKQTPlotter): Using a QScrollbar together with JKQtPlotter

This project (see ./examples/ui_bind_scrollbar/) shows how to use JKQTPlotter together with a QScrollBar for panning.

The source code of the main application can be found in ui_bind_scrollbar.cpp.

First we create a QWidget, a plot and a QScrollBar in a layout:

    QWidget win;
    QVBoxLayout* lay=new QVBoxLayout();
    win.setLayout(lay);

    JKQTPlotter* plot=new JKQTPlotter(&win);
    lay->addWidget(plot);
    // add a QScrollBar below the plot
    QScrollBar* scroll=new QScrollBar(Qt::Horizontal, &win);
    scroll->setMinimum(0);
    scroll->setMaximum(100);
    scroll->setPageStep(10);
    lay->addWidget(scroll);

Then we add a plot ranging from x=0 to x=100, with 10000 datapoints:

    // 4. create a graph
    JKQTPDatastore* ds=plot->getDatastore();
    const int NPOINTS=10000;
    JKQTPXYLineGraph* graph=new JKQTPXYLineGraph(plot);
    const size_t colX=ds->addLinearColumn(NPOINTS, 0, 100, "x");
    graph->setXColumn(colX);
    graph->setYColumn(ds->addCalculatedColumnFromColumn(colX, [](double x) { return 10.0*sin(x*3.0)*fabs(cos((x/8.0))); }, "f(x)"));
    graph->setDrawLine(true);
    graph->setSymbolType(JKQTPNoSymbol);
    // ... add the graphs to the plot, so it is actually displayed
    plot->addGraph(graph);

The plot axis range is limited to the plot range and zooming in y-direction is disabled

    plot->setAbsoluteXY(0,100,-10,10);
    // show everything in y-direction
    plot->setY(-10,10);
    // fix y-range, so no zoming occurs in y
    plot->getYAxis()->setRangeFixed(true);

Now we need a slot for the QScrollBar (here implemented as a functor), which calculates the desired view in the graph and sets it. Note the blockSignals()-calls that stop the plot from sending signals back to the scrollbar, which would cause a strange loop due to the fact that the scrollbar only accepts integers for range and value!

    QObject::connect(scroll, &QScrollBar::valueChanged, [plot,scroll](int value) {
        const double scrollRange=scroll->maximum()-scroll->minimum()+scroll->pageStep();
        const double scrollPos=scroll->value();
        const double scrollPageSize=scroll->pageStep();
        const double scrollRelative=scrollPos/scrollRange;
        const double plotFullRange=plot->getAbsoluteXMax()-plot->getAbsoluteXMin();
        const double plotViewRange=scrollPageSize/scrollRange*plotFullRange;
        const double plotViewStart=plot->getAbsoluteXMin()+scrollRelative*plotFullRange;
        plot->blockSignals(true);
        plot->setX(plotViewStart, plotViewStart+plotViewRange);
        plot->blockSignals(false);
    });
    scroll->setValue(1); // ensure to call slot once!

A second functor catches whenever the user zooms or pans (or otherwise changes the axis ranges) of the plot by catching the signal JKQTPlotter::zoomChangedLocally() and from that calculates the value (and pageStep) for the QScrollBar:

    QObject::connect(plot, &JKQTPlotter::zoomChangedLocally, [scroll](double newxmin, double newxmax, double newymin, double newymax, JKQTPlotter* plot) {
        const double plotFullRange=plot->getAbsoluteXMax()-plot->getAbsoluteXMin();
        const double plotViewRange=newxmax-newxmin;
        const double plotRelViewRange=(plotViewRange>=plotFullRange)?1.0:(plotViewRange/(plotFullRange-plotViewRange));
        const double plotRelViewStart=(newxmin-plot->getAbsoluteXMin())/plotFullRange;
        const double scrollRange=scroll->maximum()-scroll->minimum();
        scroll->blockSignals(true);
        scroll->setPageStep(plotRelViewRange*scrollRange);
        scroll->setValue(plotRelViewStart*scrollRange);
        scroll->blockSignals(false);
    });

The window from this example looks like this:

With the same method, it is also possible to add x- and y-scrollbars: