Violin Plot Maker

How the Violin Plot Maker works

Violin plot maker: what to paste

Two layouts work. The wide layout has a header row naming each group and one column of numbers beneath it; columns can be different lengths and blank cells are skipped. The long layout has exactly two columns, a group name and a value, one row per observation. Separate cells with commas or tabs, so a block copied from a spreadsheet pastes straight in. Each group needs at least two numbers, and the shape only becomes trustworthy from a few dozen.

How the shape is drawn

The curve is a kernel density estimate: a small Gaussian bump is placed on every value and the bumps are added up, giving a smooth estimate of how densely values sit at each point on the axis. The bandwidth of the bumps follows Silverman's rule of thumb from the spread and count of the group, so groups with more data get finer detail. Smoothing scales that bandwidth: Narrow shows more bumps and possible multiple peaks, Wide gives a rounder shape that hides noise. The curve is cut off just beyond the smallest and largest values so it does not suggest data that is not there.

Width and the inner mark

Same width stretches every violin to the same maximum width, which makes shapes easy to compare but hides the fact that one group may be far more concentrated than another. Scaled to density uses one shared scale, so a tight group appears fat and a spread-out group thin. Inside each violin, Box draws a miniature box plot with the median as a white dot and the inter-quartile range as a bar, the usual convention; Quartile lines draws the three quartiles across the shape; Points overlays every observation with a little sideways jitter; Nothing leaves the outline clean for a slide.

Reading a violin plot

A single bulge with the median near its centre is a roughly symmetric distribution. A bulge near one end with a long tail towards the other is skew. Two bulges mean the group is really a mix of two populations, which a box plot would hide completely; that is the main reason to prefer a violin. The width at any point is relative density, not a count, so compare shapes rather than reading areas as totals.

Exporting

Download SVG gives a vector file with one path per violin and editable text. Download PNG renders at one, two or three times the preview width. Copy SVG puts the markup on the clipboard.

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