If you take my chart and extend it to May, then the trend of undecideds breaking for the incumbent remains. Some examples:
May 1972: Nixon 53, McGovern 34. Result: Nixon 61, McGovern 37
May 1976: Carter 53, Ford 40. Result: Carter 50, Ford 48
May 1980: Carter 40, Reagan 32. Result: Reagan 51, Carter 41
May 1984: Reagan 50, Mondale 46. Result: Reagan 59, Mondale 41
May 1988: Dukakis 54, Bush 38. Result: Bush 53, Dukakis 46
May 1992: Bush 35, Challengers 60. Result: Bush 38, Challengers 62
May 1996: Clinton 47, Dole 32. Result: Clinton 49, Dole 41
May 2000: Bush 52, Gore 37. Result: Gore 48, Bush 48
Cool deal. Thanks!