It also should be noted that the map seems misleadingly optimistic in practice for Republican Senators in at least several cases.
For example this listing of the most vulnerable Senators has Pat Toomey of Pennsylvania as the 3rd most vulnerable Senator. Pennsylvania is simply a rather blue state at this point especially in Presidential election years.
http://atr.rollcall.com/10-vulnerable-senators-2016/
Not even putting it in the toss-up category seems to clearly understate how much at risk Toomey actually is.
The same listing has Richard Burr of North Carolina as the 7th most vulnerable Senator, even though the map in question actually states it is a likely Republican hold. By contrast, the Democratic Senator in the leans Democratic list (Michael Bennett of Colorado) is actually only rated 10 in the list I linked to. Basically Burr has lousy poll numbers and the state went for Obama in the 2008 election with it still basically remaining in the swing state category.
The listing also has Ray Blunt of Missouri as the 8th most vulnerable Senator even though the map has it him likely Republican hold. Essentially its a case of particularly strong Democratic challenger and a incumbent Senator in not an especially strong position. While the state actually did go barely for Obama in 2008, its probably fair to say it is a case where the inherent leanings of the state would still help the incumbent so the challenger is likely going to have to outperform the Presidential vote to actually win though. (Although it should be noted the state has voted in Democrats in circumstances where it went Republican in Presidential races in the past.)
Basically the big picture is the map really tends to understate the vulnerability of some Republican Senators and how many of them are truly potentially vulnerable in this election. My assessment is especially given the political trends of recent years, most scenarios where Hillary wins the Presidency will see at least a 50/50 Senate if not an even more favorable outcome for Democrats. This is especially true if the Republicans stick with refusing to confirm a candidate until after the election which should definitely be a political problem for Republican incumbents and even just candidates in blue leaning states and swing states or various sorts. (I expect Obama to pick a nominee which makes this position especially problematic for Republicans to defend.)