var width = 400,
    height = 500,
    shiftKey;

var svg = d3.select("body")
    .attr("tabindex", 1)
    .on("keydown.brush", keydown)
    .on("keyup.brush", keyup)
    .each(function() { this.focus(); })
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    .attr("width", width)
    .attr("height", height);

var link = svg.append("g")
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var brush = svg.append("g")
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var node = svg.append("g")
    .attr("class", "node")
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  graph.links.forEach(function(d) {
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    d.target = graph.nodes[d.target];
  });

  link = link.data(graph.links).enter().append("line")
      .attr("x1", function(d) { return d.source.x; })
      .attr("y1", function(d) { return d.source.y; })
      .attr("x2", function(d) { return d.target.x; })
      .attr("y2", function(d) { return d.target.y; });

  brush.call(d3.svg.brush()
        .x(d3.scale.identity().domain([0, width]))
        .y(d3.scale.identity().domain([0, height]))
        .on("brushstart", function(d) {
          node.each(function(d) { d.previouslySelected = shiftKey && d.selected; });
        })
        .on("brush", function() {
          var extent = d3.event.target.extent();
          node.classed("selected", function(d) {
            return d.selected = d.previouslySelected ^
                (extent[0][0] <= d.x && d.x < extent[1][0]
                && extent[0][1] <= d.y && d.y < extent[1][1]);
          });
        })
        .on("brushend", function() {
          d3.event.target.clear();
          d3.select(this).call(d3.event.target);
        }));

var force = d3.layout.force()
        .charge(-120)
        .linkDistance(30)
        .nodes(graph.nodes)
        .links(graph.links)
        .size([width, height])
        .start();

node = node.data(graph.nodes).enter().append("circle")
      .attr("r", 4)
      .attr("cx", function(d) { return d.x; })
      .attr("cy", function(d) { return d.y; })
      .on("mousedown", function(d) {
        if (!d.selected) { // Don't deselect on shift-drag.
          if (!shiftKey) node.classed("selected", function(p) { return p.selected = d === p; });
          else d3.select(this).classed("selected", d.selected = true);
        }
      })
      .on("mouseup", function(d) {
        if (d.selected && shiftKey) d3.select(this).classed("selected", d.selected = false);
      })
      .call(d3.behavior.drag()
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                  node.filter(function(d) { return d.selected; })
                  .each(function(d) { d.fixed |= 2; })
                  })
              .on("drag", function(d1) {
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                  .each(function(d) {
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                          d.y += d3.event.dy;

                          d.px += d3.event.dx;
                          d.py += d3.event.dy;
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                  force.resume();
                  })
              .on("dragend", function(d) {
                  node.filter(function(d) { return d.selected; })
                  .each(function(d) { d.fixed &= ~6; })
                  }));

      function tick() {
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              .attr("y1", function(d) { return d.source.y; })
              .attr("x2", function(d) { return d.target.x; })
              .attr("y2", function(d) { return d.target.y; });

          node.attr('cx', function(d) { return d.x; }).attr('cy', function(d) { return d.y; });

      };

      force.on("tick", tick);


function keydown() {
  shiftKey = d3.event.shiftKey || d3.event.metaKey;
}

function keyup() {
  shiftKey = d3.event.shiftKey || d3.event.metaKey;
}