By "plot" do you mean "chart" or "graph"? Plot by itself is probably the wrong word to use in this context, mostly because it has multiple meanings and can be confusing -- at first I thought you were talking about something like the plot of a movie.
There are a number of words that mean to add information that makes something (a sentence, an explanation, a chart, a diagram, whatever) more difficult to understand, but the one that jumps to mind is obfuscate:
obfuscate (v): Make obscure, unclear, or unintelligible.
The scale bar on the side is unnecessary because it only obfuscates the significance of the chart.
Alternately, if the bar only makes the chart more complicated, you could say it distracts or detracts from the main plot of the chart, or it clutters (up) the chart, or that it simply makes the chart harder to read.
The scale bar on the side is a visual distraction that clutters the chart without adding any significant information.
The scale bar makes the chart harder to read. I understand why you thought it was necessary, but you really should remove it.
Note: "Obfuscate" is one of those "big" words that may be appropriate to the context but can sound pretentious. Only a relatively small percentage of English speakers will know what it means. Sometimes it's better to keep it simple -- "confuse" is a good, if rough, synonym.