D3 tooltip using SVG title element

<!DOCTYPE html>
<html>
  <head>
    <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/d3@2.10.3/d3.v2.js"></script>
  </head>
  <body>
    <div id="chart"></div>
    <script type="text/javascript">

var w = 800, h = 500;      
      var colors = d3.scale.category20();

var vis = d3.select("#chart").append("svg:svg")
          .attr("width", w)
          .attr("height", h);

var data = d3.range(20).map(function() { return { x:Math.random()*w, y:Math.random()*h, r:Math.random()*30 } });

vis.selectAll("circle")
         .data(data)
       .enter().append("svg:circle")
         .attr("stroke", "black")
         .attr("fill", function(d, i) { return colors(i); })
         .attr("cx", function(d, i) { return d.x; })
         .attr("cy", function(d, i) { return d.y; })
         .attr("r", function(d, i) { return d.r; })
        // Here we add an SVG title element the contents of which is effectively rendered in a tooltip   
       .append("svg:title")
          .text(function(d, i) { return "My color is " + colors(i); });

</script>

</body>
</html>

Modified http://mbostock.github.com/d3/d3.js to a secure url

https://mbostock.github.com/d3/d3.js