I believe I understand your question. If you want to change the time span for each interval on your chart you can define that through the "Define report content" page.
When defining your report, from the "Define report content" page, click "Define data through a form." Define your report type, and each of the values in the drop down boxes. You can then click "Set span for time axis intervals..." and specify your interval of every month, 2 months, etc.
If that does not answer your questions please clarify what you would like to do and provide the search string.
now we have december, so if i look now at the graph i would like to see the word "December" below the graph.
And if i look at it next month, than i would like to see "January".
Like %b or %B for the strftime() function.