order month and year value in X-axis

Hi All.

I created function to display X-axisvalue format such "Jan, 2015". In original month and year are integer numbers. When I use month field of cause order is OK. But when I use the function as axis value I'm loosing of order. How do I display function value, for instance, from Jan 2015 thru Dec 2015, left to right in the correct sequence?

My function look like:

MonthName({Graph.Month}, true)& ', ' &ToText({Graph.Year},0,"");

How to fix the problem?

Thanks