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function getAvg(arr) {
const total = arr.reduce((acc, c) => acc + c, 0);
return total / arr.length;
}
function getSum(arr) {
return arr.reduce((acc, c) => acc + c, 0);
}
function trendline(data, xKey, yKey) {
const xData = data.map((value) => value[xKey]);
const yData = data.map((value) => value[yKey]);
// average of X values and Y values
const xMean = getAvg(xData);
const yMean = getAvg(yData);
// Subtract X or Y mean from corresponding axis value
const xMinusxMean = xData.map((val) => val - xMean);
const yMinusyMean = yData.map((val) => val - yMean);
const xMinusxMeanSq = xMinusxMean.map((val) => Math.pow(val, 2));
const xy = [];
for (let x = 0; x < data.length; x++) {
xy.push(xMinusxMean[x] * yMinusyMean[x]);
}
// const xy = xMinusxMean.map((val, index) => val * yMinusyMean[index]);
const xySum = getSum(xy);
// b1 is the slope
const b1 = xySum / getSum(xMinusxMeanSq);
// b0 is the start of the slope on the Y axis
const b0 = yMean - b1 * xMean;
return {
slope: b1,
yStart: b0,
calcY: (x) => b0 + b1 * x,
};
}
const averageDelta = ([x,...xs]) => {
if (x === undefined)
return NaN
else
return xs.reduce(
([acc, last], x) => [acc + (x - last), x],
[0, x]
) [0] / xs.length
}
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