Fix ggplot2 ‘Groups with fewer than two datapoints’ warning in mcmc_nuts_treedepth()#412

jgabry merged 2 commits intostan-dev:masterstan-dev/bayesplot:masterfrom

BjarkeHautop:fix-ggplot2-treedepthBjarkeHautop/bayesplot:fix-ggplot2-treedepthCopy head branch name to clipboard