Chart diff of two values

Hello,
I would like to create chart that will show percentage of target that is met.

I have aggregate value Sum of contracts.

I would like to show on chart Sum of contracts/Target in percent.

I tried to use this https://community.skuid.com/t/superbank-chart-options-data-preprocess?topic-reply-list…  but it does not work.

I suppose that I need too use before render snippet. Could you help me?

Best regards
Pawel