Fluctuate means to change continually; shift back and forth; vary irregularly. The price of gold fluctuated wildly over the past week. The price of gold fluctuated between $1,000 and $1,200 yesterday.

To me, "fluctuated" carries a very definite feeling of irregular movement both up and down. The idea of "up AND down" is very strong. So I don't believe something can "fluctuate down," I would phrase this as "the proportion of the audience fluctuated around a gradual downward trend..." "Proportion of audience" is wrong. If we are talking about a television audience choosing between, let's say, election news and a crime drama, the specific phrases that apply are "audience share" or "rating." ("Share" means the relative proportion, a percentage; "rating" is a measure of the absolute number of people watching). In any case, "proportion of audience" is grammatically wrong in itself. One can say "the proportion of the audience" or, more naturally, "the percentage of the audience." One can also say "the audience percentage."

Hello Mai, "in" sounds much more natural than "on" in your sentence. It also makes more sense since the progression the graph depicts followed or moved in an overall downward trend; "in" implies an adherence to an overall direction regardless of smaller fluctuations. Hope that helps!

So fluctuated to me implies that the proportion of the audience both went up in number and down in number and it seems odd to use it specifically with a downward trend. While a slightly different meaning, you could use "the proportion of the audience fluctuated, with a gradual downward trend...". Otherwise, I think I would usually use "fluctuated around." Edit: In rereading context, I'm sticking with "fluctuated around" although I'm keeping the earlier comment because I think it's probably useful to someone.

in, but 'the proportion of audience' is wrong. Do you mean the size of the audience?