Grouped bar chart

Side-by-side comparison of 2–5 series within each X-band — e.g. revenue per region per quarter. Each outer band (a quarter) is subdivided into one inner band per series (a region).

Demo — Apollo crewed missions per year, 1968–1972

The plain-bar variant (?chart=bar) below plots crewed Apollo missions flown per calendar year. 1969 carries four flights (Apollo 9 LM rehearsal, Apollo 10 lunar reconnaissance, Apollo 11 first landing, Apollo 12 second landing) — the program's single most ambitious year. By 1972 only Apollo 17 flew.

Source: Apollo program — Wikipedia

Grouped variant — Apollo annual budget by NASA centre

The same Apollo dataset, sliced differently. Annual program outlays ($B, 1973 dollars) for five reference years (1962, 1964, 1966, 1969, 1972), grouped by the three centres that owned the spend: Marshall (Saturn V), Manned Spacecraft Center (CSM + LM), and Kennedy (launch ops + tracking). The 1966 peak matches the late-stage Saturn V flight-hardware build right before Apollo 4's all-up test.

Source: Apollo program — Costs — Wikipedia

Public surface

The grouped layout is one extra encoding on top of the standard bar chart:

let plot = Plot::new(rows)
    .mark(Mark::Bar { value_labels: false })
    .encode(plot::x("quarter", ScaleKind::Band))
    .encode(plot::y("revenue", ScaleKind::Linear))
    .encode(plot::color("region"))
    .encode(plot::x_offset("region"));

Adding plot::x_offset(field) re-bands the X axis: each unique value of field becomes a sub-band within the outer X band.

Layout

Info

The outer X band (e.g. "Q1") spans some pixel range [bx0, bx1]. The grouped layout constructs an inner BandScale over the distinct XOffset categories with that range as its pixel range and a 10% inner padding, then asks for the sub-band's [ix0, ix1].

Pairing with Legend

When the same column drives both Color and XOffset, Plot::legend returns a legend whose colours match the bar segments exactly. Use the legend chapter for placement.

Theme integration

Grouped bars reuse the bar palette + theme. Inner band padding is fixed at 0.10 today; an explicit PlotTheme.bar_inner_padding field lands when the value needs to be customisable.