Hmm, maybe then use 3 axes? 1 for the number of countries you can enter, one for the area of the country you can enter, and 1 for length of visa (maybe not just

Hmm, maybe then use 3 axes? 1 for the number of countries you can enter, one for the area of the country you can enter, and 1 for length of visa (maybe not just length though). Take the line of best fit, see which passport is furthest above that line?