Bands

Use QAccelPlot.BandSeries to fill the area between a low and a high value at each X: confidence and prediction intervals, tolerance masks, and min/max or mean ± σ envelopes. Draw the center line with a separate LineCurve.

Panning and zooming a band costs nothing on the CPU, whatever its size. Dashed edge lines are the exception: zooming recomputes their dash positions in one pass over the data points.

Draw a band around a line

QAccelPlot.Plot {
    id: plot
    xAxis: QAccelPlot.Axis { viewportMin: 0; viewportMax: 10 }
    yAxis: QAccelPlot.Axis { viewportMin: 0; viewportMax: 10 }

    QAccelPlot.BandSeries {
        name: "90% interval"
        xAxis: plot.xAxis
        yAxis: plot.yAxis
        color: "#4065b5ff"
        Component.onCompleted: setData([0, 5, 10], [2, 4, 3], [4, 7, 8])
    }

    QAccelPlot.LineCurve {
        name: "Forecast"
        xAxis: plot.xAxis
        yAxis: plot.yAxis
        color: "#65b5ff"
        Component.onCompleted: setData([Qt.point(0, 3), Qt.point(5, 5.5), Qt.point(10, 5.5)])
    }
}

Declare the band before the line so the line is drawn on top. Nested intervals are separate bands, widest first.

At each data point the band spans from the smaller to the larger of low and high, so swapped values still fill correctly.

The default legend shows a Fill swatch in the band's color.

Edge lines

Set edges.width to draw lines along the lower and upper edges:

QAccelPlot.BandSeries {
    edges.width: 1
    edges.color: "#b065b5ff"
    edges.lineStyle: QAccelPlot.DashLine { pattern: [4, 3] }
}

Property	Default	Notes
edges.width	0	No edge lines at 0.
edges.color	invalid	Uses the band's color at full opacity.
edges.lineStyle	SolidLine	DashLine and NoLine also work.

Feed data from C++

Data points are interleaved (x, low, high) triples. The data API matches LineCurve with three values per data point:

auto dataPoints = std::vector<double>{};
dataPoints.reserve(count * 3);
for (auto i = 0; i < count; ++i) {
    dataPoints.insert(dataPoints.end(), {x[i], mean[i] - 2 * sigma[i], mean[i] + 2 * sigma[i]});
}
band->setData(std::move(dataPoints), count);

For float buffers, worker-thread handoff, and data bounds, see Select the data path.

• setData() keeps doubles and uploads them relative to an origin near the viewport, so epoch timestamps stay precise. setDataF() stores floats.
• postData() hands a buffer over from any thread without copying it.
• setData(xs, lows, highs) takes three separate arrays, from QML or C++.
• appendData(x, low, high) adds one data point.

Invalid data points

A data point is invalid when x, low, or high is NaN or ±Inf, or is not strictly positive on a logarithmic axis. The fill and the edge lines leave a gap there. Auto-ranging skips invalid values one by one.

Hover

hovered is true while the mouse pointer is inside the band or on an edge line. Only the topmost series under the mouse pointer is hovered, so nested bands highlight one at a time. Set hoverEnabled: false on a series drawn over a band to leave hover to the band:

QAccelPlot.BandSeries {
    color: hovered ? "#8065b5ff" : "#4065b5ff"
    edges.width: hovered ? 1 : 0
}

Limits

At most 16,777,216 (2^24) data points are drawn, fewer on GPUs whose maximum texture size is below 6144. Data points beyond the limit are not drawn or hovered, and a warning is logged once.

Complete source: examples/plot_types/bands