How to display a portion of a graph - RRDI 2.0.1

Here is what I would like to do, very simple but I did not sort it out :

I do have a graph (x,y) where x represents a date and y represents a data. This graphs start in 2011 and ends in 2013 so it is too large when displayed and cannot be exploid. The value of y is rising during the 3 years.

So I would like to display only the last six month from today with the right offset of the y value. The problem that I do have is that when I do display the graph on the last six months the value of y is set to zéro and does not take into account the value of y six months ago from the full graph (3 years).

How do I extract and display a portion of an existing graph without starting the y value to 0 ??