Manga are indeed graphic novels, but yes these days western YA/adult graphic novel sales are very low compared to manga sales (even excluding in Japan!) and childrens graphic novels. Though in fairness to Scott Pilgrim that 1 million figure is North America alone and right after the movie came out. Current total sales would be much, much higher.
Anyway heres source material less popular than Scott Pilgrim that got adapted a bunch: Snowpiercer. The comic is fairly obscure but it got a major film, a sequel graphic novel and a TV show.
I just decided to change this sig.
Blaaaaaaargh
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