Set the initial zoom level of a brush.

This examples demonstrates how to use D3's brush component to implement focus + context zooming. Click and drag in the small chart below to pan or zoom.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

svg {
  font: 10px sans-serif;
}

path {
  fill: steelblue;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.brush .extent {
  stroke: #fff;
  fill-opacity: .125;
  shape-rendering: crispEdges;
}

</style>
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script>

var margin = {top: 10, right: 10, bottom: 100, left: 40},
    margin2 = {top: 430, right: 10, bottom: 20, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom,
    height2 = 500 - margin2.top - margin2.bottom;

var parseDate = d3.time.format("%b %Y").parse;

var x = d3.time.scale().range([0, width]),
    x2 = d3.time.scale().range([0, width]),
    y = d3.scale.linear().range([height, 0]),
    y2 = d3.scale.linear().range([height2, 0]);

var xAxis = d3.svg.axis().scale(x).orient("bottom"),
    xAxis2 = d3.svg.axis().scale(x2).orient("bottom"),
    yAxis = d3.svg.axis().scale(y).orient("left");

var brush = d3.svg.brush()
    .x(x2)
    .on("brush", brushed);

var area = d3.svg.area()
    .interpolate("monotone")
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.price); });

var area2 = d3.svg.area()
    .interpolate("monotone")
    .x(function(d) { return x2(d.date); })
    .y0(height2)
    .y1(function(d) { return y2(d.price); });

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom);

svg.append("defs").append("clipPath")
    .attr("id", "clip")
  .append("rect")
    .attr("width", width)
    .attr("height", height);

var focus = svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var context = svg.append("g")
    .attr("transform", "translate(" + margin2.left + "," + margin2.top + ")");

d3.csv("sp500.csv", function(error, data) {

data.forEach(function(d) {
    d.date = parseDate(d.date);
    d.price = +d.price;
  });

var dateExtent = d3.extent(data.map(function(d) { return d.date; }));

x.domain(dateExtent);
  y.domain([0, d3.max(data.map(function(d) { return d.price; }))]);
  x2.domain(x.domain());
  y2.domain(y.domain());
  brush.extent([new Date(dateExtent[1].valueOf() - 1000 * 365 * 24 * 60 * 60),  dateExtent[1]]);

focus.append("path")
      .datum(data)
      .attr("clip-path", "url(#clip)")
      .attr("d", area);

focus.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

focus.append("g")
      .attr("class", "y axis")
      .call(yAxis);

context.append("path")
      .datum(data)
      .attr("d", area2);

context.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height2 + ")")
      .call(xAxis2);

context.append("g")
      .attr("class", "x brush")
      .call(brush)
    .selectAll("rect")
      .attr("y", -6)
      .attr("height", height2 + 7);

brushed();
  brush.event(context.select('g.x.brush'));

});

function brushed() {
  x.domain(brush.empty() ? x2.domain() : brush.extent());
  focus.select("path").attr("d", area);
  focus.select(".x.axis").call(xAxis);
}

</script>

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

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