Of course, in the film version of Return of the Jedi, we only knew the character as the Emperor. As such, in books Palpatine was only referred to as "the Emperor" until the late 1980s. After Timothy Zahn's Heir to the Empire trilogy were published in the early 1990s, fans became more familiar with the Emperor's actual name and started to refer to him as Emperor Palpatine.
With the Prequel Trilogy, we started to see more references to "Senator" and "Chancellor" Palpatine and relatively fewer references to "Emperor Palpatine". However, "Emperor Palpatine" is still the most frequent reference for Palpatine until 2002, after Attack of the Clones came out. At that point, both "Darth Sidious" and "Chancellor Palpatine" become more frequent, but the difference only becomes significant after Revenge of the Sith. Google N-grams Viewer only covers up to 2008, so this doesn't even cover the Clone Wars cartoon series, which of course contains many more references to "Chancellor Palpatine"!
One word of warning: Google N-grams currently only covers books. It doesn't include comics, such as Dark Empire, which of course features Palpatine in quite a central role. However, it does seem that, for much of the written Star Wars media, Palpatine is now primarily viewed as a Sith Lord, not as a politician.