Fixed aspect ratio

Hello, i want to plot a 2d histogram with fixed axpect ratio between the x and the y axes. There is a way to do that? Up to now i'm only able to do that setting the canvas size empirically and test the dimensions of the histogram contained. The other solution looks to calculate all the margin and set the dimensions of the canvas taking them in account.

Thanks. Ciao

-- 
Daniele

   Physics is like sex. Sure, it may give some practical results, but
   that's not why we do it.                     -- Richard P. Feynman