This is on Mac OS-X 10.10.3, Chrome Version 77.0.3865.90
Go to the Calendar page (- TrainerRoad) and open the TSS graph. The X-axis covers two year’s worth of data. Hit refresh and the X-axis now covers the entire period covered by your Strava activities. The only way to go back to the two year span is to leave the page and come back.
I first started on TR in 2014 so that’s the first peak. I’ve added historical data to Strava going back to 1970 that’s used by a data analysis tool which is why you see so many years.