How about you use the relative_time to calculate when it happened and then chart using it. Let's say that the dates above are coming in a field called test in epoch time then try something like:
your query to return events earliest="-21d"
| eval myTime=case(test >= relative_time(now(), "-7d"), "CurrentWeek", test >= relative_time(now(), "-14d") AND test < relative_time(now(), "-7d", "PriorWeek", test >= relative_time(now(), "-21d") AND test < relative_time(now(), "-14d", "ThirdWeek", 1=1, "Other")
| stats count("100_ of Ads Viewed") AS Total by myTime
Assumption above is "-7d" means "seven days prior to now" and not exactly "Current Week starting from Monday"
If test field has date as a string then take out the epoch time from it first, using strptime , before the case statement in above search as follows:
...| eval epochTest=strptime(test, "%Y-%m-%d")
| eval myTime=case(epochTest >= relative_time(now(), "-7d"), "CurrentWeek",... and so on
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