How to create skill tree?

I would like to create a skill tree for my own and a community usage. I hav开发者_Go百科e data in format like below:

skill_1 [description etc.] requires: none
skill_2 [...] requires: skill_1
skill_3 [...] requires: skill_1, skill_2
skill_4 [...] requires: skill_1 OR skill_2

(data format from Civilization 5 units promotion)

now i want to change those informations into graphic, tree-like structure. Something like: skill tree image

The main problem I have is that connections between skills tangle and I would recive mess instead of clear, helpful picture, when I add those skills just one by one. Is there any (sort?) algorithm to prevent (or minimalise) crossing connections? Or I have to possition them mannually in a graphic program?

Try an automatic graph layout program like GraphViz.