Invalid center coordinates when a point/polygon annotation is configured using x/yMin and y/xMax

[stockiNail]

When a point/polygon annotation is configured using x/yMin and y/xMax, the calculated centerX and centerY are wrong because the center point of result box is used. This is correct only if the box is a square.

chartjs-plugin-annotation/src/helpers/helpers.chart.js

Lines 140 to 149 in 76ba0e9

	return {
	x: box.x + options.xAdjust,
	y: box.y + options.yAdjust,
	x2: box.x + size + options.xAdjust,
	y2: box.y + size + options.yAdjust,
	centerX: box.centerX + options.xAdjust,
	centerY: box.centerY + options.yAdjust,
	width: size,
	height: size
	};

To use the center point of the box, it should be:

    const adjustCenterX = box.centerX + options.xAdjust;
    const adjustCenterY = box.centerY + options.yAdjust;
    return {
      x: adjustCenterX - radius,
      y: adjustCenterY - radius,
      x2: adjustCenterX + radius,
      y2: adjustCenterY + radius,
      centerX: adjustCenterX,
      centerY: adjustCenterY,
      width: size,
      height: size
    };