Not only is it linear; it also appears to only "turn on" once the percentage of straight-party Republican voting hits 20%. The logic there may have been that in precincts that are already voting overwhelmingly Democrat, there weren't enough Trump votes to steal to make it worth the risk of detection. And conversely, that the more heavily a precinct was voting Trump, the more Trump votes (in percentage terms) they could safely steal without arousing suspicion.