&quot;Sichuan&quot; is how the name is romanized using Pinyin, which is now more or less the standard romanization for Mandarin. &quot;Szechuan&quot; is from the older Wade-Giles system, and I think &quot;Szechwan&quot; is just a variation. The rise of the &quot;Sichuan&quot; spelling on your graph roughly corresponds to the rise of Pinyin.