They were all on the right side, so what I'd gather from that chart is that in 2008 Kucinich and Bonner were the only real liberal candidates.
But that's not the point. Trying to stick someone on a scale or rate them by numbers or whatever have you is complete bullshit. I could say I want to sterilize all the gays and amend the constitution to impose a state religion and if I were still good on welfare and gun control end up solidly in the middle of the democratic party on a lot of scales.
So, please tell me how we are supposed to know how a candidate is more or less liberal, oh Smart One?
Why do we need to? What actually matters is their stance in individual issues. Look at those rather than some abstracted average. Granted that's a bit too much effort for most voters. Most voters would rather just have someone tell them, "This is the politician that will vote how you want them to... most likely." Like some sort of online dating service rather than taking the effort to look up their history and stances on their own.. I mean yeah, it'll take an hour or so for the big votes, president and such. A few more for the congressional votes... a few days probably to find out state politicians... and hell, weeks if you care enough to figure out if a local judge is worth keeping or not. I don't even know when you get to the fiddly little local positions like... tax collector or coroner... But I think at the very least we should be spending the few hours every 2-4 years to figure out or president and congress votes. Hell, most of the time with congress, if you're happy with your incumbent you can just do a quick scan of the competition.
Instead we're just going "eh, whatever, they're (insert my party name)" they're fine. Or we hear a name on tv some surrounding language and it gets implanted in our heads that they're too liberal or too conservative for our tastes and we'll vote anyone else in to avoid them.