Simple graph with grid lines in v4

This is a simple graph demonstrating the addition of grid lines in conjunction with a line graph. This was written using d3.js v4 and is a follow on to the simple graph example here.

This graph is part of the code samples for the update to the book D3 Tips and Tricks to version 4 of d3.js.

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 2px;
}

.grid line {
  stroke: lightgrey;
  stroke-opacity: 0.7;
  shape-rendering: crispEdges;
}

.grid path {
  stroke-width: 0;
}

</style>
<body>

<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

// set the dimensions and margins of the graph
var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// parse the date / time
var parseTime = d3.timeParse("%d-%b-%y");

// set the ranges
var x = d3.scaleTime().range([0, width]);
var y = d3.scaleLinear().range([height, 0]);

// define the line
var valueline = d3.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.close); });

// append the svg obgect to the body of the page
// appends a 'group' element to 'svg'
// moves the 'group' element to the top left margin
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 + ")");

// gridlines in x axis function
function make_x_gridlines() {		
    return d3.axisBottom(x)
        .ticks(5)
}

// gridlines in y axis function
function make_y_gridlines() {		
    return d3.axisLeft(y)
        .ticks(5)
}

// Get the data
d3.csv("data.csv", function(error, data) {
  if (error) throw error;

// format the data
  data.forEach(function(d) {
      d.date = parseTime(d.date);
      d.close = +d.close;
  });

// Scale the range of the data
  x.domain(d3.extent(data, function(d) { return d.date; }));
  y.domain([0, d3.max(data, function(d) { return d.close; })]);

// add the X gridlines
  svg.append("g")			
      .attr("class", "grid")
      .attr("transform", "translate(0," + height + ")")
      .call(make_x_gridlines()
          .tickSize(-height)
          .tickFormat("")
      )

// add the Y gridlines
  svg.append("g")			
      .attr("class", "grid")
      .call(make_y_gridlines()
          .tickSize(-width)
          .tickFormat("")
      )

// add the valueline path.
  svg.append("path")
      .data([data])
      .attr("class", "line")
      .attr("d", valueline);

// add the X Axis
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

// add the Y Axis
  svg.append("g")
      .call(d3.axisLeft(y));

});

</script>
</body>

https://d3js.org/d3.v4.min.js