Definition of fences in boxplot

LordFinzenz · August 21, 2019, 8:54am

How are lower and upper fences computed in px.box()? I can’t find anything about that in the documentation which is strange, as there are multiple ways to define the fences/whiskers…

etienne · August 21, 2019, 2:15pm

Here’s how they’re computed (in JavaScript):

github.com

plotly/plotly.js/blob/2aec9efd4237b7ef7f38d9f0f237f410820db7b0/src/traces/box/calc.js#L99-L114


// lower and upper fences - last point inside
// 1.5 interquartile ranges from quartiles
cdi.lf = Math.min(
    cdi.q1,
    boxVals[Math.min(
        Lib.findBin(2.5 * cdi.q1 - 1.5 * cdi.q3, boxVals, true) + 1,
        bvLen - 1
    )]
);
cdi.uf = Math.max(
    cdi.q3,
    boxVals[Math.max(
        Lib.findBin(2.5 * cdi.q3 - 1.5 * cdi.q1, boxVals),
        0
    )]
);

In words, the lower fence is the smallest value in the sample that’s smaller than Q1 but greater than (Q1 + 1.5*IQR) and similarly for the upper fence.

LordFinzenz · August 21, 2019, 2:31pm

You mean “smaller than Q1 but greater than (Q1 - 1.5*IQR)” for the lower fence right?

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Definition of fences in boxplot

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