mcmc_pairs doesn't show max_treedepth warnings for rstan models

[dmphillippo]

mcmc_pairs doesn't behave in the same way as rstan::pairs with respect to highlighting max_treedepth warnings. As a result, setting the max_treedepth option in mcmc_pairs to the same value passed to rstan does not show any iterations exceeding max_treedepth.

This is because mcmc_pairs only flags iterations where treedepth__ > max_treedepth: https://github.com/stan-dev/bayesplot/blob/42f7b514ea196e89c735be9075d486403a28f381/R/mcmc-scatterplots.R#L354

whereas rstan::pairs flags iterations where treedepth__ >= max_treedepth: https://github.com/stan-dev/rstan/blob/71ab1409531c2c5e412635482fee545c77e2a070/rstan/rstan/R/pairs.R#L78

The same condition is used in rstan to provide the max_treedepth warning after fitting a model: https://github.com/stan-dev/rstan/blob/da2fc9c079534a82d3d26adda51ad17bf22f5e2b/rstan/rstan/R/check_hmc_diagnostics.R#L49

For example, here's a trivial model fitted in rstan with max_treedepth = 2 to get lots of max_treedepth warnings

m <- stan_model(model_code = 'parameters {real y;} model {y ~ normal(0,1);}')
mfit <- sampling(m, iter = 1000, control = list(max_treedepth = 2))
#> Warning messages:
#> 1: There were 777 transitions after warmup that exceeded the maximum treedepth. Increase max_treedepth above 2. See
#> http://mc-stan.org/misc/warnings.html#maximum-treedepth-exceeded 
#> 2: Examine the pairs() plot to diagnose sampling problems

mcmc_pairs gives us:

mcmc_pairs(mfit, 
           np = nuts_params(mfit), 
           lp = log_posterior(mfit), 
           max_treedepth = mfit@stan_args[[1]]$control$max_treedepth,
           condition = pairs_condition(nuts = "accept_stat__"))

Whereas rstan::pairs gives us:

pairs(mfit, pars = c("y", "lp__"))

This also affects downstream packages using bayesplot::mcmc_pairs like rstanarm and multinma, which set max_treedepth in mcmc_pairs from the stan control argument in this manner, and as a result don't seem to plot max_treedepth warnings.

I think a simple fix would be to change > to >= here: https://github.com/stan-dev/bayesplot/blob/42f7b514ea196e89c735be9075d486403a28f381/R/mcmc-scatterplots.R#L354

I'm happy to create a PR, if you agree.