Compare panels and show ranges

Use facets to compare the same measurements across groups. Add error bars or a ribbon when your data includes lower and upper bounds. The plot draws these supplied values; calculating a confidence interval or another statistical summary is a separate step.

This example uses illustrative measurement bounds, not fitted confidence intervals:

import std.plot as gg
import std.io
import std.bytes
from std.plot import Column, PlotError
from std.bytes import BytesError

def main() -> Result[(), AllocError | IoError | PlotError | BytesError]:
    data = gg.table([
        ("day", Column.Num([1.0, 2.0, 3.0, 1.0, 2.0, 3.0]?)),
        ("value", Column.Num([2.0, 4.0, 3.0, 3.0, 5.0, 4.0]?)),
        ("lower", Column.Num([1.0, 2.0, 1.5, 2.0, 3.0, 2.5]?)),
        ("upper", Column.Num([3.0, 6.0, 4.5, 4.0, 7.0, 5.5]?)),
        ("site", Column.Text(["North", "North", "North",
                             "South", "South", "South"]?)),
    ]?)?
    chart = gg.ggplot(data, gg.aes("day", "value", ymin="lower", ymax="upper"), [
        gg.geom_ribbon(fill="blue", alpha=0.2),
        gg.geom_errorbar(colour="navy", width=0.15),
        gg.geom_line(colour="navy"),
        gg.geom_point(colour="navy"),
        gg.facet_wrap("site", ncol=2),
        gg.labs(title="Measurement ranges by site", x="Day", y="Value"),
        gg.theme_minimal(),
    ]?)?
    mut output = std.io.cursor(std.bytes.empty()?)
    notes = chart.write_svg(&mut output, width=10.0, height=4.0)?
    print(notes, output.data.to_text()?.startswith("<svg"))?
    Ok(())

[] True

Fixed axes let you compare heights directly between sites. Choose scales=gg.Scales.FreeY when each panel needs its own vertical range, but then compare the axis values rather than the visual heights. With two grouping variables, use gg.facet_grid(rows="region", cols="period").

A reference layer whose table omits site appears in every panel. Give it its own mapping with inherit_aes=False when it should not inherit aesthetics whose columns are absent; see combine plot layers. Histogram counts and bar stacks remain independent within each panel.

Both interval endpoints contribute to the axes. Missing bounds omit an error bar and interrupt a ribbon at that x; inspect the returned notes with real data. geom_linerange draws the same interval without caps. To display horizontal intervals, add coord_flip.

The larger output size gives two panels room for their labels. All panels are checked before writing SVG, so a chart that cannot fit reports an error instead of hiding labels. See facets and uncertainty and intervals for the full contracts and current limits.