use constant gap for rank plots (#2180)

* use constant gap rank plot

* update changelog

* Update arviz/plots/backends/matplotlib/traceplot.py

Co-authored-by: Oriol Abril-Pla <oriol.abril.pla@gmail.com>

* Update CHANGELOG.md

Co-authored-by: Oriol Abril-Pla <oriol.abril.pla@gmail.com>

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Fix `reloo` outdated usage of `ELPDData` ([2158](https://github.com/arviz-devs/arviz/pull/2158))
 - plot_bpv smooth discrete data only when computing u_values ([2179](https://github.com/arviz-devs/arviz/pull/2179))
 - Fix bug when beanmachine objects lack some fields ([2154](https://github.com/arviz-devs/arviz/pull/2154))
+- Fix gap for `plot_trace` with option `kind="rank_bars"` ([2180](https://github.com/arviz-devs/arviz/pull/2180))
 
 ### Deprecation
 
@@ -27,6 +28,7 @@
 ### Maintenance and fixes
 - Fix dimension ordering for `plot_trace` with divergences ([2151](https://github.com/arviz-devs/arviz/pull/2151))
 
+
 ## v0.13.0 (2022 Oct 22)
 
 ### New features

arviz/plots/backends/matplotlib/rankplot.py

@@ -71,7 +71,7 @@ def plot_rank(
         all_counts = np.empty((len(ranks), len(bin_ary) - 1))
         for idx, row in enumerate(ranks):
             _, all_counts[idx], _ = histogram(row, bins=bin_ary)
-        gap = all_counts.max() * 1.05
+        gap = 2 / ranks.size
         width = bin_ary[1] - bin_ary[0]
 
         bar_kwargs.setdefault("width", width)

arviz/plots/backends/matplotlib/traceplot.py

@@ -192,6 +192,9 @@ def plot_trace(
     fill_kwargs = matplotlib_kwarg_dealiaser(fill_kwargs, "fill_between")
     rug_kwargs = matplotlib_kwarg_dealiaser(rug_kwargs, "scatter")
     rank_kwargs = matplotlib_kwarg_dealiaser(rank_kwargs, "bar")
+    if compact:
+        rank_kwargs.setdefault("bar_kwargs", {})
+        rank_kwargs["bar_kwargs"].setdefault("alpha", 0.2)
 
     textsize = plot_kwargs.pop("textsize", 10)