Polybrush Example

Here's a d3 plugin that allows you to create a polygon selection. You instantiate it just like d3.svg.brush.

var brush = d3.svg.polybrush();

It has an extra public method that 'brush' does not, and that's 'isWithinExtent(x, y)'. You can use this method to test if a given point falls within the drawn extent.

if (brush.isWithinExtent(x, y)) {
  console.log("I'm inside!");
}

Usage:
Click to add an anchor point, double click to close the selection.
Drag the selection to reposition it.
Double click outside the selection to clear it.

Areas For Improvement:

Add anchor handles to allow the repositioning of an anchor point.
Better viewport edge detection. Right now it simply stops moving if you try to move the selection out of the viewport.

<!DOCTYPE html>
<html>
<head>
    <script type="text/javascript" src="https://d3js.org/d3.v2.min.js"></script>
    <script type="text/javascript" src="polybrush.js"></script>
    <style type="text/css">
    .brush .extent {
      stroke: #000;
      stroke-width: 1.5px;
      fill: #000;
      fill-opacity: 0.3;
    }
    .voronoi {
      fill: steelblue; 
      fill-opacity: 0.4;
      stroke: steelblue;
      stroke-opacity: 0.5;
    }
    .voronoi.selected {
      fill: red; 
      fill-opacity: 0.4;
      stroke: darkred;
      stroke-opacity: 0.5;
    }
    
    .point.selected {
      fill: red;
      stroke: darkred;
      stroke-width: 1;
    }
    </style>
</head>
<body>
  <div id="chart"></div>
  
  <script type="text/javascript">
  var w = 960,
      h = 500;
  var brush = d3.svg.polybrush()
    .x(d3.scale.linear().range([0, w]))
    .y(d3.scale.linear().range([0, h]))
    .on("brushstart", function() {
      svg.selectAll(".selected").classed("selected", false);
    })
    .on("brush", function() {
      // set the 'selected' class for the circle
      svg.selectAll(".point").classed("selected", function(d) {
        //get the associated circle
        var id = d3.select(this).attr("id");
        var i = id.substr(id.indexOf("-")+1, id.length);
        var vornoi = d3.select("#path-"+i);
        // set the 'selected' class for the path
        if (brush.isWithinExtent(d[0], d[1])) {
          vornoi.classed("selected", true);
          return true;
        } else {
          vornoi.classed("selected", false);
          return false;
        }
      });
    });

var vertices = d3.range(200).map(function(d) {
    return [Math.random() * w, Math.random() * h];
  });

var svg = d3.select("#chart")
    .append("svg:svg")
    .attr("width", w)
    .attr("height", h);
    
  var paths, points, clips;
  clips = svg.append("svg:g").attr("id", "point-clips");
  points = svg.append("svg:g").attr("id", "points");
  paths = svg.append("svg:g").attr("id", "point-paths");

clips.selectAll("clipPath")
    .data(vertices)
    .enter().append("svg:clipPath")
    .attr("id", function(d, i) { return "clip-"+i;})
    .append("svg:circle")
    .attr('cx', function(d) { return d[0]; })
    .attr('cy', function(d) { return d[1]; })
    .attr('r', 20);

paths.selectAll("path")
    .data(d3.geom.voronoi(vertices))
    .enter().append("svg:path")
    .attr("d", function(d) { return "M" + d.join(",") + "Z"; })
    .attr("id", function(d,i) { return "path-"+i; })
    .attr("clip-path", function(d,i) { return "url(#clip-"+i+")"; })
    .attr("class", "voronoi");

points.selectAll("circle")
    .data(vertices)
    .enter().append("svg:circle")
    .attr("class", "point")
    .attr("id", function(d, i) { return "point-"+i; })
    .attr("transform", function(d) { return "translate(" + d + ")"; })
    .attr("r", 2)
    .attr('stroke', 'none');

svg.append("svg:g")
    .attr("class", "brush")
    .call(brush);

</script>
</body>
</html>

Modified http://d3js.org/d3.v2.min.js to a secure url

https://d3js.org/d3.v2.min.js