http://myelectionanalysis.blogspot.com/2004/09/whither-undecideds.html
1996: Clinton -1, Dole +3 -- Net +4 Challenger
1992: Bush +0, Clinton -2 -- Net +2 Incumbent
1984: Reagan +2, Mondale +2 -- Net 0
1980: Carter +0, Reagan +6 -- Net +6 Challenger
1976: Ford +2, Carter +3 -- Net +1 Challenger
1972: Nixon +0, McGovern +2 -- Net +2 Challenger
1964: Johnson +5, AuH2O +3 -- Net +2 Incumbent
1956: Ike -1.5, Stevenson +1.5 -- Net +3 Challenger
1948: Truman +5.5, Dewey -4.4 -- Net +9.9 Incumbent
1944: Roosevelt +3.5, Dewey -2.5 -- Net +6 Incumbent
1940: Roosevelt +3, Wilkie -3 -- Net +6 Incumbent
1936: Roosevelt +5, Landon -7 -- Net +12 Incumbent
Here is my take on this. Since 1980, the break has gone to the Republicans (except 84 when there was no break), I believe this is because the Main stream media has made weak kneed voters uncomfortable about publicly supporting the GOP, however when they go to the booth, they will vote with their conscience.
Your chart omits a lot of relevant data, such as the fact that in years when the vote broke for the challenger, it was a blowout year. Stevenson, McGovern, Ford and Dole were wiped out. The polls were just regressing towards the mean.
This year is likely to be an under two percent year. It's also likely to be a high turnout year. I think the polls are meaningless at this point. It will be won by the party that gets its vote out.