Вывести центр окружности из части окружности

var canvas = document.createElement('canvas');
document.body.appendChild(canvas);

document.body.style.margin = 0;
canvas.style.position = 'fixed';

// get canvas 2D context and set him correct size
var ctx = canvas.getContext('2d');
resize();

let points = [];
let unclicked = false;

window.addEventListener('resize', resize);
document.addEventListener('mousemove', draw);
document.addEventListener('mousedown', setPosition);

function getApproxCircle(p) {
  // Régressions coniques, quadriques, régressions linéaires et apparentées, circulaire, sphérique
  // pp 12-13
    let Sx = 0 
    let Sy = 0 
    let Sx2 = 0 
    let Sy2 = 0 
    let Sxy = 0 
    let Sx3 = 0 
    let Sy3 = 0 
    let Sx2y = 0
    let Sxy2 = 0
    for(point of p ){
        const [x,y] = [point.x,point.y]
        const [x2,y2] = [x*x,y*y]
        const [x3,y3] = [x2*x,y2*y]
        const xy    = x*y
        const [x2y,xy2] = [xy*x,xy*y]

        Sx += x
        Sy += y
        Sx2 += x2
        Sy2 += y2
        Sxy += xy
        Sx3 += x3
        Sy3 += y3
        Sx2y += x2y
        Sxy2 += xy2
    }

    const n = points.length;
    const s11 = n * Sxy - Sx * Sy
    const s20 = n * Sx2 - Sx * Sx
    const s02 = n * Sy2 - Sy * Sy
    const s30 = n * Sx3 - Sx2 * Sx
    const s03 = n * Sy3 - Sy * Sy2
    const s21 = n * Sx2y - Sx2 * Sy
    const s12 = n * Sxy2 - Sx * Sy2
    const a = ((s30 + s12)*s02 - (s03 + s21)*s11)
            / (2*( s20 * s02 - s11 * s11));
    const b = ((s03 + s21)*s20 - (s30 + s12)*s11)
            / (2*( s20 * s02 - s11 * s11));
    const c = (Sx2 + Sy2 - 2*a*Sx - 2*b*Sy) / n;
    const radius = Math.sqrt(c + a*a + b*b);
        
    return {
        center: {
            x: a,
          y: b
        },
        radius: radius
    };
}
function setPosition(e) {
  if (unclicked && e.buttons === 1) {
    unclicked = false;
        points = [];
  }
  points.push({ x: e.clientX, y: e.clientY });
}

function resize() {
  ctx.canvas.width = window.innerWidth;
  ctx.canvas.height = window.innerHeight;
}

function clear() {
    ctx.clearRect(0, 0, window.innerWidth, window.innerHeight);
}

function draw(e) {
  if (e.buttons !== 1) { 
    unclicked = true;
    return;
  }
  
  points.push({ x: e.clientX, y: e.clientY });
    clear();  

  ctx.lineWidth = 5;
  ctx.lineCap = 'round';
  ctx.strokeStyle = '#c0392b';
  
  //point of drawn arc
  ctx.beginPath(); 
  ctx.moveTo(points[0].x, points[0].y);
  for(i=1; i<points.length; i++) {
    ctx.lineTo(points[i].x, points[i].y); 
  }
  ctx.stroke(); 
  
  let approxCircle = getApproxCircle(points);
  
  //circle
  ctx.lineWidth = 2;
  ctx.strokeStyle = '#3039cb77';
    ctx.beginPath();
  ctx.arc(approxCircle.center.x, approxCircle.center.y, approxCircle.radius, 0, 2 * Math.PI, false);
  ctx.stroke();
  
  // center of the circle
    ctx.beginPath();
  ctx.arc(approxCircle.center.x, approxCircle.center.y, 1, 0, 2 * Math.PI, false);
  ctx.fillStyle = 'green';
  ctx.fill();
  ctx.lineWidth = 5;
  ctx.strokeStyle = '#003300';
  ctx.stroke();
}