Percentage Change

Assuming you bought AAPL stock at its closing price on January 2, 2008, how would your investment fare over the next four and a half years? (Quite well, for the most part…)

Rather than simply plot the absolute price on a linear scale, which would exaggerate gains and understate losses, instead compute the relative price, i.e., the current price divided by the purchase price. Then, use a log scale to accurately show proportional gains and losses: a +100% gain (2×) is as good as a -50% loss is bad (0.5×). As long as the current value never reaches absolute zero, meaning as long as the stock is not completely worthless, the scale domain never crosses zero.

To show percentage change (+20%, -20%), simply subtract 1 from the relative price:

var formatPercent = d3.format("+.0%");

// Formats a relative price (e.g., 2) as percentage change (e.g., +100%).
function formatChange(x) {
  return formatPercent(x - 1);
}

Although the log scale ticks are adequate in relative price space, a second linear scale can also be used to generate uniformly-spaced ticks.

A nice related technique is to add color to show positive or negative change.

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

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

.axis-title {
  text-anchor: end;
}

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

.axis--x path {
  display: none;
}

.axis--y .tick:not(.tick--one) line {
  stroke-opacity: .15;
}

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 1.5px;
  stroke-linejoin: round;
  stroke-linecap: round;
}

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

var margin = {top: 30, right: 30, bottom: 40, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var formatPercent = d3.format("+.0%"),
    formatChange = function(x) { return formatPercent(x - 1); },
    parseDate = d3.time.format("%d-%b-%y").parse;

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

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

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .tickSize(-width, 0)
    .tickFormat(formatChange);

var line = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.ratio); });

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 + ")");

var gX = svg.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + height + ")");

var gY = svg.append("g")
    .attr("class", "axis axis--y");

gY.append("text")
    .attr("class", "axis-title")
    .attr("transform", "rotate(-90)")
    .attr("y", 6)
    .attr("dy", ".71em")
    .text("Change in Price");

d3.tsv("data.tsv", function(error, data) {
  if (error) throw error;

// Compute price relative to base value (hypothetical purchase price).
  var baseValue = +data[0].close;
  data.forEach(function(d) {
    d.date = parseDate(d.date);
    d.ratio = d.close / baseValue;
  });

x.domain(d3.extent(data, function(d) { return d.date; }));
  y.domain(d3.extent(data, function(d) { return d.ratio; }));

// Use a second linear scale for ticks.
  yAxis.tickValues(d3.scale.linear()
      .domain(y.domain())
      .ticks(20));

gX.call(xAxis);

gY.call(yAxis)
    .selectAll(".tick")
      .classed("tick--one", function(d) { return Math.abs(d - 1) < 1e-6; });

svg.append("path")
      .datum(data)
      .attr("class", "line")
      .attr("d", line);
});

</script>

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