Try something like this:
dc.renderAll();
function AddXAxis(chartToUpdate, displayText)
{
chartToUpdate.svg()
.append("text")
.attr("class", "x-axis-label")
.attr("text-anchor", "middle")
.attr("x", chartToUpdate.width()/2)
.attr("y", chartToUpdate.height()-3.5)
.text(displayText);
}
AddXAxis(chart1, "This is the x-axis!");
Here is a jsfiddle example: http://jsfiddle.net/djmartin_umich/x5qb9/
I hope this helps! -DJ