Q:

d3.js: Dual lines of text for each tick on an axis? 

I have a chart with dates on the x-axis. 
Now I would like to add the time aswell and place it on the row below the date for each tick because the text get rather wide for each tick with both date and time. 
By looking to the question suggestion below I tried this instead:
var xMainScale = d3.scale.linear()
   .domain([0,100])
   .range([20, 600]);

//--- date format
var formatAsDate = function(d) {var md = new Date(d*1000);
   md_utc=new Date(md.getUTCFullYear(), md.getUTCMonth(),
   md.g etUTCDate(),  md.getUTCHours(), md.getUTCMinutes(), md.getUTCSeconds());
   return md_utc.format("Y-m-d H:i:s");};

//--- label on several rows
var insertLinebreaks = function (d) {
var el = d3.select(this);
var words = d.split(' ');
el.text('');

for (var i = 0; i < words.length; i++) {
  var tspan = el.append('tspan').text(words[i]);
  if (i > 0)
    tspan.attr('x', 0).attr('dy', '15');
 }
};

//===== axis
var xMainAxis = d3.svg.axis()
  .scale(xMainScale)
  .orient("bottom")
  .ticks(5)
  .tickFormat(formatAsDate);

//====== create SVG element
var chart = d3.select("body")
  .append("svg:svg")
  .attr("width", w)
  .attr("height", h)
  .attr("id","chart")
  .attr("class","chart");

//==== main chart (there are more but I left them out here)
var main = chart.append('g')
  .attr('transform', 'translate(' + 25 + ',' + 25 + ')')
  .attr('width', w)
  .attr('height', main_h)
  .attr('class', 'main')
  .attr('id', 'mainchart');

//===== create x axis
main.append("g")
  .attr("id","main_x")
  .attr("class", "x_axis")
  .attr("transform", "translate("+0+"," + (700 - 25)  + ")")
  .call(xMainAxis);

In a function a bit down:
//----- update the axis
main.select('#main_x').call(xMainAxis);
main.selectAll('#main_x g text').each(insertLinebreaks);

I have cut and past what I think is the important parts as my code is rather large. 
I think I'm passing the wrong objects to the insertLinebreaks function, but what should I pass instead? 
/Lars

A:

Thanks to @Lars Kotthoff who helped me to find the solution. 
By changing the function insertLinebreakes to:
var insertLinebreaks = function (d) {
  var el = d3.select(this);
  var words=d3.select(this).text().split(' ');

  el.text('');

  for (var i = 0; i < words.length; i++) {
var tspan = el.append('tspan').text(words[i]);
if (i > 0)
      tspan.attr('x', 0).attr('dy', '15');
  }
 };

It works. 