Show / hide elements on mouse click with d3.js

This is a relativly straight forward graph with two lines. Each line and it's corresponding Y axis can be toggled on and off by clicking on the text labels at the bottom of the graph.

This example is based on the explanation given by 'mdml' in response to a Stack Overflow question on showing and hiding links and nodes.

It is used as an example of showing and hiding an element in d3.js using a separate element and described in the book D3 Tips and Tricks which can be downloaded for free from Leanpub.

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

body { font: 12px Arial;}

path { 
		stroke: steelblue;
		stroke-width: 2;
		fill: none;
	}

.axis path,
	.axis line {
		fill: none;
		stroke: grey;
		stroke-width: 1;
		shape-rendering: crispEdges;
	}

.legend {
		font-size: 16px;         
		font-weight: bold;         
		text-anchor: start;
		}

</style>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>

var	margin = {top: 30, right: 40, bottom: 70, left: 50}, 
	width = 600 - margin.left - margin.right,
	height = 300 - margin.top - margin.bottom;

var	parseDate = d3.time.format("%d-%b-%y").parse;

var	x = d3.time.scale().range([0, width]);
var	y0 = d3.scale.linear().range([height, 0]);
var	y1 = d3.scale.linear().range([height, 0]);

var	xAxis = d3.svg.axis().scale(x)
	.orient("bottom").ticks(5);

var	yAxisLeft = d3.svg.axis().scale(y0)
	.orient("left").ticks(5);

var	yAxisRight = d3.svg.axis().scale(y1)
	.orient("right").ticks(5);

// Get the data
data.forEach(function(d) {
	d.date = parseDate(d.date);
	d.close = +d.close;
	d.open = +d.open;
});

// Scale the range of the data
x.domain(d3.extent(data, function(d) { return d.date; }));
y0.domain([0, d3.max(data, function(d) { 
	return Math.max(d.close); })]);
y1.domain([0, d3.max(data, function(d) { 
	return Math.max(d.open); })]);

svg.append("path")
	.attr("class", "line")
	.attr("id", "blueLine")
	.attr("d", valueline(data));

svg.append("path")
	.attr("class", "line")
	.style("stroke", "red")
	.attr("id", "redLine")
	.attr("d", valueline2(data));

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

// edit the Y Axis Left
svg.append("g")	
	.attr("class", "y axis")
	.style("fill", "steelblue")
	.attr("id", "blueAxis")
	.call(yAxisLeft);

svg.append("g")
	.attr("class", "y axis")
	.attr("transform", "translate(" + width + " ,0)")
	.style("fill", "red")
	.attr("id", "redAxis")
	.call(yAxisRight);

// Add the blue line title
svg.append("text")
	.attr("x", 0)             
	.attr("y", height + margin.top + 10)    
	.attr("class", "legend")
	.style("fill", "steelblue")         
	.on("click", function(){
		// Determine if current line is visible
		var active   = blueLine.active ? false : true,
		  newOpacity = active ? 0 : 1;
		// Hide or show the elements
		d3.select("#blueLine").style("opacity", newOpacity);
		d3.select("#blueAxis").style("opacity", newOpacity);
		// Update whether or not the elements are active
		blueLine.active = active;
	})
	.text("Blue Line");

// Add the red line title
svg.append("text")
	.attr("x", 0)             
	.attr("y", height + margin.top + 30)    
	.attr("class", "legend")
	.style("fill", "red")         
	.on("click", function(){
		// Determine if current line is visible
		var active   = redLine.active ? false : true ,
		  newOpacity = active ? 0 : 1;
		// Hide or show the elements
		d3.select("#redLine").style("opacity", newOpacity);
		d3.select("#redAxis").style("opacity", newOpacity);
		// Update whether or not the elements are active
		redLine.active = active;
	})
	.text("Red Line");

</script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js