var circle = this.vis.select("svg").selectAll("circle")
      .data(this.points, function(d) { return d; });

  circle.enter().append("circle")
      .attr("class", function(d) { return d === self.selected ? "selected" : null; })
      .attr("cx",    function(d) { return self.x(d.x); })
      .attr("cy",    function(d) { return self.y(d.y); })
      .attr("r", 10.0)
      .style("cursor", "ns-resize")
      .on("mousedown.drag",  self.datapoint_drag())
      .on("touchstart.drag", self.datapoint_drag());

  circle
      .attr("class", function(d) { return d === self.selected ? "selected" : null; })
      .attr("cx",    function(d) {
        return self.x(d.x); })
      .attr("cy",    function(d) { return self.y(d.y); });

  circle.exit().remove();

hr_circles = self.graph_gps.svg.selectAll("hr_circles")
        .data(self.graph_gps.datay1); // , function(d){return d;}

hr_circles.enter().append("circle")
        .style("z-index", 3)
        .attr("class", "y1")
        .attr("r", 1)
        .attr("cx", function (d, i) {
          return xScale(d.time)
        })
        .attr("cy", function (d, i) {
          return yScale(d.vy)
        })
        .on("mouseover",
                function (d) {...displays a tooltip...})
        .on("mouseout", function (d) {
        });

  hr_circles.attr("class", "y1")
          .attr("cx", function (d, i) {
            return xScale(d.time)
          })
          .attr("cy", function (d, i) {
            return yScale(d.vy)
          })

  hr_circles.exit().remove();