AS3 :: Flex : Convert Ascii Code To Character?

I want to create a board class from canvas, which will allow to track click position on it in coordinates like A2, where A...D is Y coordinate in some scale and 1...3 is X coordinate [URL]What I want to create is a kind of convertor from canvas localX and localY to my new coordinates like A2.I am thinking of implementing if condition this way if (0.4 - x*size(from 1-3 here)/canvas.width <= X <= 0.4 + x*size(from 1-3 here)/canvas.width)
X = x;

This way I can assigned needed coordinates in X range. e.g. 1, 2 ,3 etc But what to do with alphanumeric range. (if for example I want to make it extensible)...