D3.js Set initial zoom level

Question

I have several graphs set up to zoom on the container and it works great. However, on the initial load, the zoom level is way too close. Is there a method of setting the initial zoom level to avoid having to first zoom out? I am familiar with the .scale() method but have not had any luck implementing it. Is this the way to go or is there something I am missing?

Here is what I have thus far as pertaining to zoom:

var margin = {top: 20, right: 120, bottom: 20, left: 120},
    width = 50000 - margin.right - margin.left,
    height = 120000 - margin.top - margin.bottom;

var x = d3.scale.linear()
    .domain([0, width])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([0, height])
    .range([height, 0]);

var tree = d3.layout.tree()
    .size([height, width])
    .separation(function(a, b) { return (a.parent == b.parent ? 1 : 2) / a.depth; });

var diagonal = d3.svg.diagonal()
    .projection(function(d) { return [d.x, d.y]; });

function zoom(d) {        
  svg.attr("transform",
      "translate(" + d3.event.translate + ")"+ " scale(" + d3.event.scale + ")");
}

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.right + margin.left)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")")
    .attr("pointer-events", "all")
    .call(d3.behavior.zoom()
        .x(x)
        .y(y)
        .scaleExtent([0,8])
        .on("zoom", zoom))
        .append('g');

svg.append('rect')
    .attr('width', width*5)
    .attr('height', height)
    .attr('border-radius', '20')
    .attr('fill', 'sienna');

Answer 1

D3v4 answer

If you are here looking for the same but with D3 v4,

var zoom = d3.zoom().on("zoom", zooming);

vis = svg.append("svg:svg")
     .attr("width", width)
     .attr("height", height)
     .call(zoom) // here
     .call(zoom.transform, d3.zoomIdentity.translate(100, 50).scale(0.5))
     .append("svg:g")
     .attr("transform","translate(100,50) scale(.5,.5)");