Close... but not correct. Colleridge "claimed" he had been reading Purchase's Pillgrimage after he had taken two grains of laudnum and fell into an opium stupor where he visualized the poem Kubla Khan... truth of it is a friend of Coleridge's named Perditia Robinson had taken opium while sick and had a dream where she wrote a poem about a local madman. She titled teh poem "The Idiot." When Coleridge heard the story of Perditia writing a poem while under the influence of opium and he became obsessed with the experience... and shortly thereafter claimed to have his vision where he wrote Kuble Khan. This is why Coleridge's friends always refered to Kubla Khan as his "supposed" opium vision... and hence the poem is probably based on "the idiot."