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Published
Edited
Oct 10, 2022
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detectPeaks([2, 0, 10, 2, 1])
detectPeaks([{value: 2}, {value: 0}, {value: 10}, {value: 3}, {value: 1}], d => d.value)
detectPeaks([1, 3, 2, 0, 10, 3, 1, 4], {
lookaround: 2, // the number of neighbors to compare to on each side
sensitivity: 1, // sensitivity, in terms of standard deviations above the mean
coalesce: 2 // coalesce together peaks within this distance of each other
})
function detectPeaks(data, accessor, options) {
let {lookaround, sensitivity, coalesce, full} = Object.assign({
lookaround: 2,
sensitivity: 1.4,
coalesce: 0,
full: false
}, options || accessor)
let values = typeof accessor == "function" ? data.map(accessor) : data
// Compute a peakiness score for every sample value in `data`
// We normalize the scale of the scores by mean-centering and dividing by the standard deviation
// to get a dimensionless quantity such that can be used as a sensitivity parameter
// across different scales of data (s. t. normalize(x) == normalize(k*x))
let scores = normalize(
values.map(
(value, index) => peakiness(
values.slice(max(0, index - lookaround), index),
value,
values.slice(index + 1, index + lookaround + 1)
)
)
)
// Candidate peaks are indices whose score is above the sensitivity threshold
let candidates = d3.range(scores.length).filter(index => scores[index] > sensitivity)
// If we have multiple peaks, coalesce those that are close together
let groups = candidates.length ? [[candidates[0]]] : []
d3.pairs(candidates).forEach(([a, b]) => {
if (b - a < coalesce) {
groups[groups.length - 1].push(b)
} else {
groups.push([b])
}
})
// Represent every group of peaks by the highest peak in the group
let peaks = groups.map(
group => group[d3.scan(group, (a, b) => values[b] - values[a])]
)
return full ? { data, values, scores, candidates, groups, peaks } : peaks
}
// Assigns a spikiness score to `value`, based on its left and right neighbors
peakiness = (left, value, right) => {
// assume zero outside the boundary
return value - d3.max([d3.min(left) || 0, d3.min(right) || 0]) // this can be max or mean.
}
normalize = xs => {
let mean = d3.mean(xs)
let stdev = d3.deviation(xs)
return xs.map(x => (x - mean) / stdev)
}
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Observable is your go-to platform for exploring data and creating expressive data visualizations. Use reactive JavaScript notebooks for prototyping and a collaborative canvas for visual data exploration and dashboard creation.
