adding mean and standard deviation values to chart while having % values

I am having a hard time figuring out how to visualise my mean and standard deviation
in the chart I have.

I want to have the mean and stdav visualised in the chart like in the example, however
because the values are in percentages (I want to have percentages) the mean lines always
end up at 20 % and I would like this to be changed so that the line goes where it would if the percentage values were normal number values instead.

For example the red data/IT graph has correct values but the middle line (mean line) is
above the 17,65 % bars and when I calculate the mean normally like in the left side the mean is 2,14..
which means that the line should be much further down as the 17,65 % bars represent 3 votes.

Maybe there is a simpler way to do this?