While there was a 3rd (and once, a 4th in 1984) party candidate on the ballot for all years except one (1988), none of the races could have been won for the losing candidate if all of the 3rd party votes were cast for him instead.
What is interesting as I look at the graph, are the three surges, one for Democrats in 1992, which saw an 18% jump in Democrat voters compared to the previous election (with a 22% fall in Republican voters), and two subsequent Republican surges in 2004 & 2008, with, respectively, a 30% & 17% increase over the previous election, and very little fall in Democrat votes. In 2012 there was a plummeting of Republican votes, with a 23% fall, and a slight increase in Democrat votes. On the one hand, Indiana likes to re-elect their governors (at least for the last 32 years), indicating that Pence should win a second term in 2014. On the other hand, given that Pence won with less than 50% of the vote, and his policies since then have been very unpopular, it may spell trouble, especially given the demographic shifts as the youth cohort which has been voting strongly Democrat, only continues to increase.
What would be interesting is to elucidate the factors that caused the 2004 & 2008 Republican voter surges. The 2012 drop is understandable, since the election after the incumbent leaves typically shows a significant drop, while the second year typically shows a significant increase, except for Orr's reelection in 1984.