d3 axis labeling

Axis labels aren't built-in to D3's axis component, but you can add labels yourself simply by adding an SVG text element. A good example of this is my recreation of Gapminder’s animated bubble chart, The Wealth & Health of Nations. The x-axis label looks like this:

svg.append("text")
    .attr("class", "x label")
    .attr("text-anchor", "end")
    .attr("x", width)
    .attr("y", height - 6)
    .text("income per capita, inflation-adjusted (dollars)");

And the y-axis label like this:

svg.append("text")
    .attr("class", "y label")
    .attr("text-anchor", "end")
    .attr("y", 6)
    .attr("dy", ".75em")
    .attr("transform", "rotate(-90)")
    .text("life expectancy (years)");

You can also use a stylesheet to style these labels as you like, either together (.label) or individually (.x.label, .y.label).

In the new D3js version (version 3 onwards), when you create a chart axis via d3.svg.axis() function you have access to two methods called tickValues and tickFormat which are built-in inside the function so that you can specifies which values you need the ticks for and in what format you want the text to appear:

var formatAxis = d3.format("  0");
var axis = d3.svg.axis()
        .scale(xScale)
        .tickFormat(formatAxis)
        .ticks(3)
        .tickValues([100, 200, 300]) //specify an array here for values
        .orient("bottom");