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I'd have a look at this video to give you a better idea: Pareto Charts
The reason you can't add a reference line to the categories is that you can only add a reference line to a continuous axis and your axis in currently discrete. The broad idea is that you turn this into a count of categories and then a % of total of that count to give you the 0-100% axis. This video will help though. Good luck.
Thank you, James.