"Other" bar not displayed correctly in bar chart


Sorry if the subject has already been answered, I did not find anything.
I want to use a "horizontal bar" chart to display rates for many sites (around 150), showing the worst sites first.
The "Autre" (other) section displays the results for all the sites that cannot be displayed in the chart, but it shows a summed percentage.
Would you have any solution to have it computing a global rate for all the sites (sort of average rate of all the other sites) instead ?

Many thanks for your help!