在D3s x轴刻度标签上添加工具提示

I am using the d3.svg.axis() function in order to draw the x axis of my barchart. As the labels on the axis can be very long, I need to cut them (to let's say four letters) and display the rest as tooltip. I would like to make use of svg:title, as the browser will take care of displaying the tooltip then. Any idea how I can achieve this? How can I add a title element on each label on the x axis ticks?I am using the d3.svg.axis() function in order