I am conducting an event study where I want to graph out cumulative abnormal returns surrounding the announcement of acquisition bids. The graph should look like this:

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I have a dataset with the variables company_id, date, cumulative_abnormal_return.
The cumulative abnormal returns (CAR) are computed for about 350 firms, each for a -20 to +180 window.

How do I create a graph like this, with the average CAR in the group?