I was able to brute-force an inelegant solution to this.
I created a csv of what static values that I wanted the growth to resemble. I added it as a lookup 'staticgrowth.csv'
index=myDownloads
| timechart span=1w count as ActualDownloads
| appendcols
[| inputlookup staticgrowth.csv
| eval _time=strptime(date,"%Y-%m-%d")
| eval weekBeginning=relative_time(now(), "-0w@w")
| where _time<weekBeginning
| timechart span=1week values(staticDownloadCount) as GoalDownloads]
... View more