Add more visual tests with vdiffr

[jgabry]

Here's a checklist of all of our plotting functions (broken up into MCMC and PPC and their subcategories) so we can track which do and don't yet have visual tests with vdiffr (they all have simple testthat tests, just not visual tests). @tjmahr started the process of adding visual tests in #124 and I think we should continue expand the number of our plotting functions that are tested with vdiffr.

We can use this list to keep track of the progress.

MCMC plots

MCMC-diagnostics

• mcmc_rhat()
• mcmc_rhat_hist()
• mcmc_neff()
• mcmc_neff_hist()
• mcmc_acf()
• mcmc_acf_bar()

MCMC-distributions

• mcmc_hist()
• mcmc_dens()
• mcmc_hist_by_chain()
• mcmc_dens_overlay()
• mcmc_dens_chains()
• mcmc_violin()

MCMC-intervals

• mcmc_intervals()
• mcmc_areas()
• mcmc_areas_ridges()

MCMC-nuts

• mcmc_nuts_acceptance()
• mcmc_nuts_divergence()
• mcmc_nuts_stepsize()
• mcmc_nuts_treedepth()
• mcmc_nuts_energy()

MCMC-parcoord

• mcmc_parcoord()

MCMC-recover

• mcmc_recover_intervals()
• mcmc_recover_scatter()
• mcmc_recover_hist()

MCMC-scatterplots

• mcmc_scatter()
• mcmc_hex()
• mcmc_pairs()

MCMC-traces

• mcmc_trace()
• mcmc_trace_highlight()
• mcmc_rank_overlay()
• mcmc_rank_hist()

PPC plots

PPC-discrete

• ppc_bars()
• ppc_bars_grouped()
• ppc_rootogram()

PPC-distributions

• ppc_hist()
• ppc_boxplot()
• ppc_freqpoly()
• ppc_freqpoly_grouped()
• ppc_dens()
• ppc_dens_overlay()
• ppc_ecdf_overlay()
• ppc_violin_grouped()

PPC-errors

• ppc_error_binned()
• ppc_error_hist()
• ppc_error_hist_grouped()
• ppc_error_scatter()
• ppc_error_scatter_avg()
• ppc_error_scatter_avg_vs_x()

PPC-intervals

• ppc_intervals()
• ppc_intervals_grouped()
• ppc_ribbon()

PPC-loo

• ppc_loo_pit_overlay()
• ppc_loo_pit_qq()
• ppc_loo_intervals()
• ppc_loo_ribbon()

PPC-scatterplots

• ppc_scatter()
• ppc_scatter_avg()
• ppc_scatter_avg_grouped()

PPC-test-statistics

• ppc_stat()
• ppc_stat_grouped()
• ppc_stat_freqpoly_grouped()
• ppc_stat_2d()