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According to my calculations (using US-only data from insideevs.com), Tesla has now sold more Model S than any other plug-in has sold over the last 12 months.
This data also shows that Sept also marked the first time Tesla accounted for >20% of all plug-in sales in the US over a 12mo period (19.89% as of Aug).
1917
Model S
1834
Nissan Leaf
1127
Chevy Volt
907
BMW i3
761
Ford Fusion Energi
635
Ford C-Max Energi
485
Fiat 500e
455
Toyota Prius PHV
1448
Other
9569
Total
Granted, as much as this is a reflection of the job done by Tesla, it is just as much due to the sharp declines of the Volt and Leaf, which are both being refreshed for 2016, but I still think it's an impressive feat. The Model S is now selling at nearly the same rate the Volt was at it's peak in 2013.
Note that in this chart, the axis on the left applies to Total sales (including some models not plotted on this chart), whereas all other values are on the right axis.