Ryan is the most conservative Congressmember to be nominated as VP since 1900.

[retromike22]

http://fivethirtyeight.blogs.nytimes.com/2012/08/11/a-risky-rationale-behind-romneys-choice-of-ryan/

"Various statistical measures of Mr. Ryan peg him as being quite conservative. Based on his Congressional voting record, for instance, the statistical system DW-Nominate evaluates him as being roughly as conservative as Representative Michele Bachmann, the controversial congresswoman of Minnesota.

By this measure, in fact, which rates members of the House and Senate throughout different time periods on a common ideology scale, Mr. Ryan is the most conservative Republican member of Congress to be picked for the vice-presidential slot since at least 1900. He is also more conservative than any Democratic nominee was liberal, meaning that he is the furthest from the center. (The statistic does not provide scores for governors and other vice-presidential nominees who never served in Congress.)"

[retromike22]

I'm sorry, the ideological classification of vice-presidents that you have posted is completely absurd. How exactly is John Nance Garner the most liberal vice-president on that list?

What part of the data do you take issue with? Those ideology scores aren't just pulled out of thin air. They're DW-NOMINATE scores which are used in political science as the gold standard for comparing the ideologies of legislators. Garner's score there is an objective measure based on his voting behavior relative to the rest of his congress (taking into account every roll call vote in a given congress).

DW-NOMINATE does a very poor job of comparing people from different eras.  Also, they may label their axis as being Liberal-Conservative, but it really is more of a Democratic-Republican axis than Liberal-Conservative.

Hence the argument that the chart is a useful measure of partisanship rather than anything else. I think Silver's point regarding Ryan stands if we're talking about partisanship rather than ideology per se, though.

In that case, Ryan is the vp choice who was least likely to vote against his party.