Canvas + D3: 20K points with mouseover

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Canvas Scatterplot</title>
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<script src="https://d3js.org/d3.v4.js"></script>
	<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>
	<script src="https://d3js.org/d3-scale.v1.min.js"></script>
	
	<style type="text/css">
		.hiddenCanvas{
			display: none;
		}

div#tooltip {
			position: absolute;        
			display: inline-block;
			padding: 10px;
			font-family: 'Open Sans' sans-serif;
			color: #000;
			background-color: #fff;
			border: 1px solid #999;
			border-radius: 2px;
		    pointer-events: none;
			opacity: 0;
			z-index: 1;
		}
	</style>

</head>
<body>
<div id="container"></div>
<div id="tooltip"></div>

var width = 1400,
		height = 750;

// main canvas
	var mainCanvas = d3.select('#container')
		.append('canvas')
		.classed('mainCanvas', true)
		.attr('width', width)
		.attr('height', height);

// hidden canvas
	var hiddenCanvas = d3.select('#container')
		.append('canvas')
		.classed('hiddenCanvas', true)
		.attr('width', width)
		.attr('height', height);

var customBase = document.createElement('custom');
	var custom = d3.select(customBase); // replacement of SVG

// map to track color the nodes.
	var colorToNode = {};
	// function to create new colors for picking
	var nextCol = 1;
	function genColor(){
		var ret = [];
		if (nextCol < 16777215){
			ret.push(nextCol & 0xff); //R
			ret.push((nextCol & 0xff00) >> 8); //G
			ret.push((nextCol & 0xff0000) >> 16); //B

nextCol += 1;
		}
		var col = "rgb(" + ret.join(',') + ")";
		return col;
	}

var x = d3.scaleLinear()
		.domain([2, 8])
		.range([0, width]);

var y = d3.scaleLinear()
		.domain([2,8])
		.range([height, 0]);

databind(data, x, y);
		var t = d3.timer(function(elapsed){
			draw(mainCanvas, false);
			if (elapsed > 300) t.stop();
			// timer running the draw function repeatedly for 300ms.
		});

function databind(data, x, y){
		var join = custom.selectAll('custon.circle')
			.data(data);

join
			.merge(enterSel)
			.transition()
			.attr('fillStyleHidden', function(d){
				if (!d.hiddenCol){
					d.hiddenCol = genColor();
					colorToNode[d.hiddenCol] = d;
				}
				return d.hiddenCol;
			});

}

function draw(canvas, hidden){
		var context = canvas.node().getContext('2d');

context.clearRect(0, 0, width, height);

var elements = custom.selectAll('custom.circle');
		elements.each(function(d,i){
			var node = d3.select(this);
			context.fillStyle = hidden ? node.attr('fillStyleHidden') : 'steelblue';
			context.beginPath();
			context.arc(node.attr('x'), node.attr('y'), node.attr('r'), 0, 2*Math.PI);
			context.fill();
		})
	}

d3.select('.mainCanvas').on('mousemove', function(){
		draw(hiddenCanvas, true);
		var mouseX = d3.event.layerX || d3.event.offsetX;
		var mouseY = d3.event.layerY || d3.event.offsety;

var hiddenCtx = hiddenCanvas.node().getContext('2d');
		var col = hiddenCtx.getImageData(mouseX, mouseY, 1, 1).data;

var colKey = 'rgb(' + col[0] + ',' + col[1] + ',' + col[2] + ')';
		var nodeData = colorToNode[colKey];

if (nodeData){
			console.log(nodeData);
			d3.select('#tooltip')
				.style('opacity', 0.8)
				.style('top', d3.event.pageY + 5 + 'px')
				.style('left', d3.event.pageX + 5 + 'px')
				.html('x: ' + nodeData.x + '<br>' + 'y: ' + nodeData.y + '<br>' + 'radius: ' + nodeData.r);
		} else {
			d3.select('#tooltip')
				.style('opacity', 0);
		}

})
</script>
	
</body>
</html>

Modified http://d3js.org/d3.v4.js to a secure url

https://d3js.org/d3.v4.js

https://d3js.org/d3-scale-chromatic.v1.min.js

https://d3js.org/d3-scale.v1.min.js