Does anyone know how to transform the y-axis to a logarithmic scale when creating line-graphs in Illustrator?

I don't understand. Can you clarify what you're trying to achieve in detail? Preferably, by providing an image that illustrates it.

I don't think Illustrator's (anaemic) graph features support a logarithmic value axis. I see no option for it in any graph (scatter and line bing the most appropriate). Searching the help files turns up nothing.