The graph shows the latest scores to be released by Voteview. Unlike conventional interest group ratings, they take into account all votes in the Senate (except those in which less than 5% of the Senate dissented from the majority).
The points show the estimates on the scale with most positive being more conservative. The lines around the points are 95% confidence intervals (i.e. the actual ideology of the senator has a 19 in 20 probability of falling on the line).
Bernie Sanders is the most liberal member of the Senate. Rand Paul and Ted Cruz are literally off the charts as the two most conservative senators. Fellow Candidate Marco Rubio is the seventh most Republican while Lindsey Graham is the fifth most liberal.