After all, let's look at history and go all the way back to 1992 (a date that Miller does not include in his graphic). By June of 1992, Bill Clinton's approval rating could have been better, as S.E. Cupp reminds us:
In June 1992, Clinton was third in the polls, behind President George Herbert Walker Bush and Ross Perot, and his favorability rating was an embarrassing 16%. Even forgetting that Perot split the conservative vote and probably cost Bush reelection, it’s hard to believe that Clinton went from such low favorability to the White House in such a short span of time.
And these early favorability numbers might not be the most persuasive. According to Miller's chart, the candidate with the highest net favorability in the March of an election year was Bob Dole. Another fact from history: there was no President Dole.