[BUG]: Violin plot width incorrectly calculated in g... | plotly/plotly.js

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Description

When using violinmode: 'group' with violins at multiple x-axis categories, but without offsetgroup set, the width calculation uses the total number of traces across the entire plot instead of the number of traces at each x-position. This causes excessive empty space even when gap parameters are set to 0.

Screenshots/Video

Steps to reproduce

<!DOCTYPE html>
<html>
<head>
	<!-- Load plotly.js into the DOM -->
	<script src='https://cdn.plot.ly/plotly-3.1.2.js'></script>
</head>
<body>
    <div id="myDiv"></div>
    <script>
        // Create data: 2 models × 3 evaluations = 6 traces total
        // But at each x-position, there are only 2 violins

        const models = ["Model A", "Model B"];
        const evals = ["Eval 1", "Eval 2", "Eval 3"];
        const colors = ["blue", "red"];

        const traces = [];

        models.forEach((model, modelIdx) => {
            evals.forEach((evalName, evalIdx) => {
                const yData = Array.from({length: 50}, () =>
                    Math.random() * 2 - 1 + modelIdx
                );

                traces.push({
                    type: 'violin',
                    x: Array(50).fill(evalName),
                    y: yData,
                    name: model,
                    legendgroup: model,
                    scalegroup: model,
                    // offsetgroup: model, // Uncomment to fix spacing
                    showlegend: evalIdx === 0,
                    line: { color: colors[modelIdx] }
                });
            });
        });

        const layout = {
            violinmode: 'group',
            violingap: 0,      // No gap between categories
            violingroupgap: 0, // No gap between groups
            title: 'Bug Demo: Excessive empty space despite gap=0'
        };

        Plotly.newPlot('myDiv', traces, layout);
    </script>
</body>
</html>

Notes

Root Cause

I think the issue is in src/traces/box/cross_trace_calc.js.

When traces are split across multiple x-categories (each trace appears at only one x-value) but offsetgroup is not set, the formula incorrectly uses the global trace count numTotal:

var num = nOffsetGroups || numTotal;
...
bdPos = dPos * groupFraction * groupGapFraction / num;

This should divide by the number of traces at each specific x-position, not the total number of traces in the plot.

Workaround

Uncommenting offsetgroup: model, below scalegroup: model, in the above code produces the expected behaviour with no space between violins. See the below image:

However I don't believe this should be required - certainly it isn't mentioned in the docs.

Inizia leggendo src/traces/box/cross_trace_calc.js e riproduci l’esempio con sei tracce violin, la modalità raggruppata e senza offsetgroup. Confronta l’allocazione della larghezza a ogni posizione x con l’attuale calcolo globale delle tracce, quindi verifica che la spaziatura delle tracce violin raggruppate corrisponda alle impostazioni con intervallo zero e valuta se la logica condivisa dei box-plot è interessata.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.

[BUG]: Violin plot width incorrectly calculated in grouped mode with multiple x-categories

Mô tả

Description

Screenshots/Video

Steps to reproduce

Notes

Root Cause

Workaround

Related

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[BUG]: Violin plot width incorrectly calculated in grouped mode with multiple x-categories

Mô tả

Description

Screenshots/Video

Steps to reproduce

Notes

Root Cause

Workaround

Related

Hướng dẫn đóng góp

Bắt đầu từ đâu

Hướng nghiên cứu

Đánh giá

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