Mouseover demonstration. Modified from the original to use the official d3.js URL.

<html>
	<head>
		<meta charset="utf-8">
		<title>Mouse Over and Out with Moving Infobox</title>
		<script type="text/javascript" src="https://d3js.org/d3.v3.js"></script>
		<link type="text/css" rel="stylesheet" href="style.css">
	</head>
	<body>
		
		
		
		
		<div class="infobox">
			<p>Here is where we will say something.</p>
		</div>

// this will be ran whenever we mouse over a circle
			var myMouseOverFunction = function() {
				var circle = d3.select(this);
				circle.attr("fill", "red" );

// show infobox div on mouseover.
				// block means sorta "render on the page" whereas none would mean "don't render at all"
				d3.select(".infobox").style("display", "block");	
				// add test to p tag in infobox
				d3.select("p").text("This circle has a radius of " + circle.attr("r") + " pixels.");
			}

var myMouseOutFunction = function() {
				var circle = d3.select(this);
				circle.attr("fill", "steelblue" );
				
				// display none removes element totally, whereas visibilty in last example just hid it
				d3.select(".infobox").style("display", "none");	
			}

var myPositionFunction = function() {
				return Math.random() * w;
			}
			
			// this should be called when the mouse is moved
			// we will attach it to our svg area so that it detects mouse movement on our entire visualization
			// we are trying to get the infobox to move with the mouse
			var myMouseMoveFunction = function() {
				// save selection of infobox so that we can later change it's position
				var infobox = d3.select(".infobox");
				// this returns x,y coordinates of the mouse in relation to our svg canvas
				var coord = d3.svg.mouse(this)
				// now we just position the infobox roughly where our mouse is
				infobox.style("left", coord[0] + 15  + "px" );
				infobox.style("top", coord[1] + "px");
			}
			
			var chart = d3.select("body")
				.append("svg:svg")
				.attr("width", w)
				.attr("height", h)
				.on('mousemove', myMouseMoveFunction); // here we attach our function which updates info box position
				// we attach it here, because we want it to work everywhere on our graphic
			
			chart.selectAll("circle").data(data)

.enter().append("svg:circle")

.attr("cx", myPositionFunction)

.attr("cy", myPositionFunction)

.attr("r", function(d) {		
					return d;
				})

.attr("fill", "steelblue")

.on("mouseover", myMouseOverFunction)

.on("mouseout", myMouseOutFunction);  	
		</script>
	</body>
</html>

Modified http://d3js.org/d3.v3.js to a secure url

https://d3js.org/d3.v3.js