Local Variables

In response to mbostock's block: Local Variables and the Tweet I implemented a little variation having a .local(name[, value) method you can use on selections.

On set, the value is stored on the given element:

element.local("foo", value);

On get, the value is retrieved from given element, or the nearest ancestor that defines it:

var value = element.local("foo");

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

body {
  font: 10px sans-serif;
  margin: 0;
}

.line {
  fill: none;
  stroke: #666;
  stroke-width: 1.5px;
}

.area {
  fill: #e7e7e7;
}

text {
  text-anchor: end;
}

</style>
<body>
<script src="//d3js.org/d3.v4.0.0-alpha.44.js"></script>
<script src="d3-local.js"></script>
<script>

var margin = {top: 8, right: 10, bottom: 2, left: 10},
    width = 960 - margin.left - margin.right,
    height = 69 - margin.top - margin.bottom;

var parseDate = d3.timeParse("%b %Y");

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

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

var symbols = d3.nest()
      .key(function(d) { return d.symbol; })
      .entries(data);

x.domain([
    d3.min(symbols, function(symbol) { return symbol.values[0].date; }),
    d3.max(symbols, function(symbol) { return symbol.values[symbol.values.length - 1].date; })
  ]);

svg.append("path")
      .attr("class", "area")
      .attr("d", function(d) {
      	return d3.select(this).local("area")(d.values)
      });
  
  svg.append("path")
      .attr("class", "line")
      .attr("d", function(d) { 
      	return d3.select(this).local("line")(d.values);
      });
  
  svg.append("text")
      .attr("x", width - 6)
      .attr("y", height - 6)
      .text(function(d) { return d.key; });
});

function type(d) {
  d.price = +d.price;
  d.date = parseDate(d.date);
  return d;
}

</script>

https://d3js.org/d3.v4.0.0-alpha.44.js