D3 Bar Chart Update Pattern

This animated bar chart is adapted from Mike Bostock's sample bar chart to allow redrawing and animated transitions.

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

.bar {
  fill: steelblue;
}

.bar:hover {
  fill: brown;
}

.axis {
  font: 10px sans-serif;
}

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

.x.axis path {
  display: none;
}

</style>
<body>
<script src="d3.v3.min.js"></script>
<script>

// Mike Bostock "margin conventions"
var margin = {top: 20, right: 20, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// D3 scales = just math
// x is a function that transforms from "domain" (data) into "range" (usual pixels)
// domain gets set after the data loads
var x = d3.scale.ordinal()
    .rangeRoundBands([0, width], .1);

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

// D3 Axis - renders a d3 scale in SVG
var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .ticks(10, "%");

// create an SVG element (appended to body)
// set size
// add a "g" element (think "group")
// annoying d3 gotcha - the 'svg' variable here is a 'g' element
// the final line sets the transform on <g>, not on <svg>
var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")

svg.append("g")
    .attr("class", "y axis")
  .append("text") // just for the title (ticks are automatic)
    .attr("transform", "rotate(-90)") // rotate the text!
    .attr("y", 6)
    .attr("dy", ".71em")
    .style("text-anchor", "end")
    .text("Frequency");

// d3.tsv is a wrapper around XMLHTTPRequest, returns array of arrays (?) for a TSV file
// type function transforms strings to numbers, dates, etc.
d3.tsv("data.tsv", type, function(error, data) {
  replay(data);
});

function type(d) {
  // + coerces to a Number from a String (or anything)
  d.frequency = +d.frequency;
  return d;
}

function draw(data) {
  // measure the domain (for x, unique letters) (for y [0,maxFrequency])
  // now the scales are finished and usable
  x.domain(data.map(function(d) { return d.letter; }));
  y.domain([0, d3.max(data, function(d) { return d.frequency; })]);

// another g element, this time to move the origin to the bottom of the svg element
  // someSelection.call(thing) is roughly equivalent to thing(someSelection[i])
  //   for everything in the selection\
  // the end result is g populated with text and lines!
  svg.select('.x.axis').transition().duration(300).call(xAxis);

// same for yAxis but with more transform and a title
  svg.select(".y.axis").transition().duration(300).call(yAxis)

// THIS IS THE ACTUAL WORK!
  var bars = svg.selectAll(".bar").data(data, function(d) { return d.letter; }) // (data) is an array/iterable thing, second argument is an ID generator function

bars.exit()
    .transition()
      .duration(300)
    .attr("y", y(0))
    .attr("height", height - y(0))
    .style('fill-opacity', 1e-6)
    .remove();

// data that needs DOM = enter() (a set/selection, not an event!)
  bars.enter().append("rect")
    .attr("class", "bar")
    .attr("y", y(0))
    .attr("height", height - y(0));

// the "UPDATE" set:
  bars.transition().duration(300).attr("x", function(d) { return x(d.letter); }) // (d) is one item from the data array, x is the scale object from above
    .attr("width", x.rangeBand()) // constant, so no callback function(d) here
    .attr("y", function(d) { return y(d.frequency); })
    .attr("height", function(d) { return height - y(d.frequency); }); // flip the height, because y's domain is bottom up, but SVG renders top down

}

</script>