在鼠标悬停在画布上的形状/位置（坐标）上时显示工具提示

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;
function reOffset() {
  var BB = canvas.getBoundingClientRect();
  offsetX = BB.left;
  offsetY = BB.top;
}
var offsetX, offsetY;
reOffset();
window.onscroll = function (e) {
  reOffset();
};
window.onresize = function (e) {
  reOffset();
};

var hotspots = [
  { x: 100, y: 100, radius: 20, tip: "You are over 100,100" },
  { x: 100, y: 200, radius: 20, tip: "You are over 100,200" },
];

draw();

$("#canvas").mousemove(function (e) {
  handleMouseMove(e);
});

function draw() {
  for (var i = 0; i < hotspots.length; i++) {
    var h = hotspots[i];
    ctx.beginPath();
    ctx.arc(h.x, h.y, h.radius, 0, Math.PI * 2);
    ctx.closePath();
    ctx.stroke();
  }
}

function handleMouseMove(e) {
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  mouseX = parseInt(e.clientX - offsetX);
  mouseY = parseInt(e.clientY - offsetY);

  ctx.clearRect(0, 0, cw, ch);
  draw();
  for (var i = 0; i < hotspots.length; i++) {
    var h = hotspots[i];
    var dx = mouseX - h.x;
    var dy = mouseY - h.y;
    if (dx * dx + dy * dy < h.radius * h.radius) {
      ctx.fillText(h.tip, h.x, h.y);
    }
  }
}

body{ background-color: ivory; }
#canvas{border:1px solid red; margin:0 auto; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Hover over a hotspot circle</h4>
<canvas id="canvas" width=300 height=300></canvas>