There is no perfect comparison and there never will be. Having said that, YTD would at least smooth out the month to month fluctuations caused by Tesla's supply and shipment schedule.

Arguably a reasonable approach might be to throw away the month that Tesla had its highest deliveries but still use the sum of all other months.

There’s a ton of options to make the stats better comparable. What I primarily wanted to point out is that an arbitrary cutoff point skews the stats. For any cutoff point you choose to make your point I can pick another. An interesting comparison would be the 2nd,3rd,4th month of sales of the 2019 leaf vs the model 3. But the leaf isn’t shipping to Europe yet afaics.