6 hour marks with d3's time axis.

The tick format provided by d3.time.scale is a multi-scale tick format, meaning that it formats times differently depending on the time. For example, the start of February is formatted as "February", while February second is formatted as "Feb 2". The format is implemented using an array of time formats, each with an associated filter. The first filter that returns true is used. (In the implementation below, the formats are processed in reverse order.) You can create your own custom multi-scale time format using the same technique with only a few lines of code.

This format prints only hours of days, rounding them to the nearest 6 hours mark.

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

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

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

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

var margin = {top: 250, right: 40, bottom: 250, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.time.scale()
    .domain([new Date(2012, 0, 1), new Date(2012, 0, 3)])
    .range([0, width]);

var xAxis = d3.svg.axis()
    .scale(x)
    .tickFormat(customTimeFormat);

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 + ")");

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

function timeFormat(formats) {
  return function(date) {
    var i = formats.length - 1, f = formats[i];
    while (!f[1](date)) f = formats[--i];
    return d3.functor(f[0])(date);
  };
}

</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

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