Currently, RadHtmlChart supports only single line of axis labels, for example you can the month numbers/names in the date axis like explained here:
The only possible option that I can suggest you in order to show labels with the years just below the months labels is to add a standard html elements outside the chart (e.g. floated divs or a table), positioning them just below the chart itself.
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.