[Hampshire] map creation software

Hi

I want to express a matrix of interconnecting data in the form of a map,
like the london underground. Does anybody know of software to do this?

I have a list of subjects which are categorised, with some related to
each other (nodes and connections) - can a piece of (free) software make
a diagram out of that matrix? I picture the categories as the
underground lines and the subjects as stations.

Many thanks

Ed