The problem with these graphs is that, in terms of the two-party system, the word “independent” has no real meaning. In the end, given the terminology of polling and pollsters, it’s just a way of dodging the issue. People say “I’m independent” as a way of saying nothing. Instead of presenting them with “nothing” as an alternative to something, the pollsters give them a way of saying nothing and then call it a “trend.”
In short, no matter what you or I might think, I promise you that most people who say “independent” have no clear idea of what they mean when they say it. In fact, they say it BECAUSE they are clueless.