[BUG]: Violin plot width incorrectly calculated in grouped mode with multiple x-categories
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描述
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.
Related
This probably also affects box plots since they share the width calculation logic in cross_trace_calc.js.
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调研方向
先阅读 src/traces/box/cross_trace_calc.js，并复现包含六条小提琴 trace、分组模式且不使用 offsetgroup 的示例。将每个 x 位置的宽度分配与当前的全局 trace 计算进行比较，然后验证分组小提琴 trace 的间距是否与零间隔设置一致，并考虑共享的 box-plot 逻辑是否受到影响。
由索引模型根据 Issue 内容生成。
评估
- 技术栈
- javascript
- 领域
- data-visualization
- Issue 类型
- 缺陷
- 难度
- 3/5
- 预计耗时
- 1-2 天
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- 58/100