Accepted answer

The translate value is aligning the graph so that x(d0) is the leftmost x value visible in the plot area. This ensures the visible portion of the plot area extends from d0 through d1 (the visible subdomain). If our full domain for the x scale has a minimum of 0, then x(0) will be shifted left (negative shift) x(d0) pixels.

I'll use a snippet to demonstrate:

var svg ="svg"),
    margin = {top: 10, right: 50, bottom: 70, left: 200},
    width = +svg.attr("width") - margin.left - margin.right,
    height = +svg.attr("height") - - margin.bottom;

// Scale for Data:
var x = d3.scaleLinear()
  .range([0, width])
// Scale for Zoom:
var xZoom = d3.scaleLinear()
var xAxis = d3.axisBottom(x).ticks(5);
var xZoomAxis = d3.axisBottom(xZoom);

var zoom = d3.zoom()
    .scaleExtent([1, 32])
    .translateExtent([[0, 0], [width, height]])
    .extent([[0, 0], [width, height]])
    .on("zoom", zoomed);
var g = svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + + ")");

// plot area
  .text("plot area");

// zoomed plot area:
var rect = g.append("rect")
// Axis for plot:
  .attr("class", "axis axis--x")
  .attr("transform", "translate(0," + height + ")")
// Axis for zoom:
  .attr("class", "axis axis-zoom-x")
  .attr("transform", "translate(0,"+(height+30)+")")
var text = g.append("text")
  .attr("y", height+60)
  .text("zoom units")
// Gratuitous intro zoom:   
var d1 = 18;
var d0 = 8;
 .call(zoom.transform, d3.zoomIdentity
 .scale(width / (x(d1) - x(d0)))
 .translate(-x(d0), 0));

function zoomed() {
  var t = d3.event.transform, xt = t.rescaleX(x);
  rect.attr("x", xt(0));
  rect.attr("width", xt(20) - xt(0));
  text.attr("x", xt(10));
<script src=""></script>
<svg width="400" height="180">
  <pattern id="stripes" patternUnits="userSpaceOnUse" width="8" height="8" patternTransform="rotate(45 0 0)">
  <rect width="3" height="8" fill="orange"></rect>


Snippet Explanation:

  • Plot area: orange stripes
  • Full scaled extent of data: grey box.
  • Left hand side of plot area is x=0 (pixels) for the parent g that holds everything.

As we zoom in the bounds of our data exceeds the plot area. We want to show a specific subdomain of our data. We achieve part of that with the scale (as you correctly deduce) but the other portion is with the translate: we push values less than the lowest value of our x subdomain to the left. By pushing the entire graph left by an amount equal to x(d0), x(d0) appears as the leftmost coordinate of the plot area.

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