regression.js

// includes regression
// https://unpkg.com/regression@1.4.0/build/regression.min.js

var canvas = document.createElement('canvas')
var ctx = canvas.getContext('2d')
document.body.appendChild(canvas)
Object.assign(canvas.style, {border:'1px solid #ccc', margin:0})


var m = 1
var c = 0
var points = []


// calculate m & c from points
function doCalculation() {
  var result = regression('linear', points)
  m = result.equation[0]
  c = result.equation[1]
}


canvas.addEventListener('mouseenter', () => points = [])
canvas.addEventListener('mousemove', (e) => {
  points.push([e.clientX, e.clientY])
  doCalculation()
  draw()
})



function draw () {
  ctx.clearRect(0,0, canvas.width, canvas.height)
  
  ctx.beginPath()
  points.forEach(([x,y]) => {
    ctx.lineTo(x,y)
  })
  ctx.strokeStyle = '#08f'
  ctx.stroke()
  
  ctx.beginPath()
  ctx.lineTo(0,c)
  ctx.lineTo(canvas.width,canvas.width*m + c)
  ctx.strokeStyle = '#f08'
  ctx.stroke()
  
  
}

https://jsbin.com/qaziqeyamu/edit?js,output