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At the end of March, Jonah Gabry and I released

bayesplot 1.5.0. The major additions

to the package were visualizations using ridgelines and a new plot for PIT

diagnostics from LOO validation. I don’t know what that LOO PIT thing is yet, so

I’ll talk about how ridgelines have been incorporated into bayesplot instead.

## Behind the scenes of `mcmc_areas()`

I’m going to start with a less obvious use of ridgelines.

`mcmc_areas()`

visualizes the posterior distribution of model parameters. It

helps us see where the peak values are. By default, it will plot the complete

range of the distribution, shade the middle 50% of the distribution, and draw a

line at the median.

For this release, I rewrote the function

so that it uses the ggridges package. I had noticed that the shapes it

produced had different areas. Here’s an example of how the plots have changed.

```
library(ggplot2)
# Temporarily install the previous version of bayesplot and
# make an mcmc_areas() plot.
withr::with_temp_libpaths({
versions::install.versions("bayesplot", "1.4.0", quiet = TRUE)
example_data <- bayesplot::example_mcmc_draws()
p_a <- bayesplot::mcmc_areas(example_data, pars = c("beta[1]", "beta[2]")) +
ggtitle("Version 1.4.0") +
bayesplot::theme_default()
})
#> package 'bayesplot' successfully unpacked and MD5 sums checked
library(bayesplot)
#> This is bayesplot version 1.5.0
#> - Plotting theme set to bayesplot::theme_default()
#> - Online documentation at mc-stan.org/bayesplot
p_b <- mcmc_areas(example_data, c("beta[1]", "beta[2]")) +
ggtitle("Version 1.5.0") +
bayesplot::theme_default()
# Show the two together
cowplot::plot_grid(p_a, p_b)
```

In the plot on the left, the shapes have different areas. Eyeballing it, it

looks like `beta[2]`

would fit inside `beta[1]`

. Area is width times height.

These shapes have different widths because they cover different values—that

makes sense. Therefore, the problem is the height—namely, the fact that the

shapes have the same height. To make the areas more similar, we adjust the

heights, and that’s what’s happening in the plot on the right. The distribution

for `beta[2]`

is concentrated to a more narrow range, and that feature manifests

in that shape’s much higher peak.

### Three plots in one

This plot is built using the ggridges packages which handles drawing the shapes

at different heights. It took a bit of trickery, however, to do the annotation

for the shaded 50% interval and the median. Here is a high-level overview of

what’s happening.

`mcmc_areas_data()`

tidies the MCMC samples into a dataframe suitable for

plotting. It computes the density of the parameter values, and it returns the

densities over *three* intervals: 1) an outer interval, 2) an inner interval to

be shaded, and 3) a narrow “point” interval.

```
library(dplyr, warn.conflicts = FALSE)
d <- mcmc_areas_data(example_data, c("beta[1]", "beta[2]")) %>%
mutate(
interval = factor(interval, c("outer", "inner", "point")))
d
#> # A tibble: 4,238 x 5
#> parameter interval interval_width x density
#>
```
#> 1 beta[1] inner 0.500 0.136 0.986
#> 2 beta[1] inner 0.500 0.137 0.988
#> 3 beta[1] inner 0.500 0.137 0.989
#> 4 beta[1] inner 0.500 0.138 0.990
#> 5 beta[1] inner 0.500 0.138 0.991
#> 6 beta[1] inner 0.500 0.139 0.992
#> 7 beta[1] inner 0.500 0.139 0.993
#> 8 beta[1] inner 0.500 0.139 0.994
#> 9 beta[1] inner 0.500 0.140 0.995
#> 10 beta[1] inner 0.500 0.140 0.997
#> # ... with 4,228 more rows

Internally, `mcmc_areas()`

uses `ggridges::geom_density_ridges()`

to draw

*three* different ridgeline plots.

```
# Separate the full range so it can be drawn on each facet.
full_range <- d %>%
filter(interval == "outer") %>%
select(-interval)
p_base <- ggplot(d) +
aes(x = x, y = parameter, fill = interval, height = density) +
# For reference include the full-range.
# `stat = "identity"` means that ggridges should not scale the heights
ggridges::geom_density_ridges(
stat = "identity", data = full_range, color = "grey80",
size = 1, fill = NA) +
ggridges::geom_density_ridges(stat = "identity") +
scale_fill_brewer(type = "seq") +
guides(fill = FALSE)
p_base +
facet_wrap("interval")
```

These intervals are layered to achieve the desired look.

There’s a bit more fussing to the actual function. For example, the width of the

intervals and whether a point is calculated are options that have to be handled.

The tips of the topmost peaks are not cut off in those plots as well. But that’s

the basic idea.

## Overlapping ridgelines for related parameters

Plots with the classic, overlapping *Unknown
Pleasures* look are supported

by

`mcmc_areas_ridges()`

. I like these plots for hierarchical models where theparameters represent similar units. In fact, part of the reason Jonah approached

me about contributing to bayesplot was a proof-of-concept

demo I wrote to visualize hierarchical

effects using ridgeline plots, back when they were popularly called “joyplots”.

Below is the “eight

schools”

example where there is a treatment effect for each school (thetas) and an

overall average effect (mu).

```
m <- shinystan::eight_schools@posterior_sample
mcmc_areas_ridges(m, pars = "mu", regex_pars = "theta") +
ggplot2::ggtitle("Treatment effect on eight schools (Rubin, 1981)")
```

If we really want to go for the *Unknown Pleasures* look, we can manually plot

the densities in a white-on-black theme. Most of the work here is padding zero

values onto the ridgelines so that they all have the same width.

```
d <- mcmc_areas_ridges_data(m, pars = "mu", regex_pars = "theta") %>%
filter(interval == "outer")
zeroes <- d %>%
# Get range of each parameter
group_by(parameter) %>%
summarise(pmin = min(x), pmax = max(x)) %>%
mutate(
min = min(pmin),
max = max(pmax),
# On each row, make a data-frame of zeroes from absolute minimum
# value to this parameter's minimum value
lower_df = purrr::map2(
min, pmin,
~ data.frame(x = seq(.x, .y, length.out = 20), density = 0)),
# Make a dataframe of zeroes from each parameter's max to the
# overall max
upper_df = purrr::map2(
pmax, max,
~ data.frame(x = seq(.x, .y, length.out = 20), density = 0)),
# Gather the zeroes together
df = purrr::map2(lower_df, upper_df, bind_rows)) %>%
select(parameter, df) %>%
tidyr::unnest(df)
ggplot(bind_rows(d, zeroes)) +
aes(x = x, y = parameter, height = density) +
ggridges::geom_density_ridges(
stat = "identity", fill = NA,
color = "grey70", scale = 5) +
theme_void() +
theme(
plot.background = element_rect(fill = "black"),
plot.margin = unit(c(.25, .25, .25, .25), units = "npc"),
axis.ticks = element_blank())
```

Finally, ridgelines appear in by-chain diagnostics using `mcmc_dens_chains()`

.

```
mcmc_dens_chains(m, pars = c("theta[1]", "theta[2]", "theta[3]"))
```

There really isn’t much interesting to say about this plot, except that it

follows Mike DeCrescenzo’s early

example of

using ridgelines on Bayesian models.

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