Single sided violin plots (enabbling split-violin plots) [WIP] - feedback welcome

CHANGELOG.md

@@ -3,10 +3,11 @@
 ## v0.x.x Unreleased
 
 ### New features
+* Add `side` argument to `plot_violin` to allow single-sided violin plots ([1996](https://github.com/arviz-devs/arviz/pull/1996))
 
 ### Maintenance and fixes
 * Add exception in `az.plot_hdi` for `x` of type `np.datetime64` and `smooth=True` ([2016](https://github.com/arviz-devs/arviz/pull/2016))
-* Change `ax.plot` usage to `ax.scatter` in `plot_pair`. ([1990](https://github.com/arviz-devs/arviz/pull/1990))
+* Change `ax.plot` usage to `ax.scatter` in `plot_pair` ([1990](https://github.com/arviz-devs/arviz/pull/1990))
 
 ### Deprecation
 

arviz/plots/backends/bokeh/violinplot.py

@@ -20,6 +20,7 @@ def plot_violin(
     shade_kwargs,
     shade,
     rug,
+    side,
     rug_kwargs,
     bw,
     textsize,
@@ -64,10 +65,9 @@ def plot_violin(
     ):
         val = x.flatten()
         if val[0].dtype.kind == "i":
-            dens = cat_hist(val, rug, shade, ax_, **shade_kwargs)
+            dens = cat_hist(val, rug, side, shade, ax_, **shade_kwargs)
         else:
-            dens = _violinplot(val, rug, shade, bw, circular, ax_, **shade_kwargs)
-
+            dens = _violinplot(val, rug, side, shade, bw, circular, ax_, **shade_kwargs)
         if rug:
             rug_x = -np.abs(np.random.normal(scale=max(dens) / 3.5, size=len(val)))
             ax_.scatter(rug_x, val, **rug_kwargs)
@@ -102,32 +102,51 @@ def plot_violin(
     return ax
 
 
-def _violinplot(val, rug, shade, bw, circular, ax, **shade_kwargs):
+def _violinplot(val, rug, side, shade, bw, circular, ax, **shade_kwargs):
     """Auxiliary function to plot violinplots."""
     if bw == "default":
         bw = "taylor" if circular else "experimental"
     x, density = kde(val, circular=circular, bw=bw)
 
-    if not rug:
+    if rug and side == "both":
+        side = "right"
+
+    if side == "left":
+        dens = -density
+    elif side == "right":
+        x = x[::-1]
+        dens = density[::-1]
+    elif side == "both":
         x = np.concatenate([x, x[::-1]])
-        density = np.concatenate([-density, density[::-1]])
+        dens = np.concatenate([-density, density[::-1]])
 
-    ax.harea(y=x, x1=density, x2=np.zeros_like(density), fill_alpha=shade, **shade_kwargs)
+    ax.harea(y=x, x1=dens, x2=np.zeros_like(dens), fill_alpha=shade, **shade_kwargs)
 
     return density
 
 
-def cat_hist(val, rug, shade, ax, **shade_kwargs):
+def cat_hist(val, rug, side, shade, ax, **shade_kwargs):
     """Auxiliary function to plot discrete-violinplots."""
     bins = get_bins(val)
     _, binned_d, _ = histogram(val, bins=bins)
 
     bin_edges = np.linspace(np.min(val), np.max(val), len(bins))
     heights = np.diff(bin_edges)
     centers = bin_edges[:-1] + heights.mean() / 2
-    right = 0.5 * binned_d
-
-    left = 0 if rug else -right
+    bar_length = 0.5 * binned_d
+
+    if rug and side == "both":
+        side = "right"
+
+    if side == "right":
+        left = 0
+        right = bar_length
+    elif side == "left":
+        left = -bar_length
+        right = 0
+    elif side == "both":
+        left = -bar_length
+        right = bar_length
 
     ax.hbar(
         y=centers,

arviz/plots/backends/matplotlib/violinplot.py

@@ -20,6 +20,7 @@ def plot_violin(
     shade,
     rug,
     rug_kwargs,
+    side,
     bw,
     textsize,
     labeller,
@@ -68,9 +69,9 @@ def plot_violin(
     for (var_name, selection, isel, x), ax_ in zip(plotters, ax.flatten()):
         val = x.flatten()
         if val[0].dtype.kind == "i":
-            dens = cat_hist(val, rug, shade, ax_, **shade_kwargs)
+            dens = cat_hist(val, rug, side, shade, ax_, **shade_kwargs)
         else:
-            dens = _violinplot(val, rug, shade, bw, circular, ax_, **shade_kwargs)
+            dens = _violinplot(val, rug, side, shade, bw, circular, ax_, **shade_kwargs)
 
         if rug:
             rug_x = -np.abs(np.random.normal(scale=max(dens) / 3.5, size=len(val)))
@@ -101,21 +102,29 @@ def plot_violin(
     return ax
 
 
-def _violinplot(val, rug, shade, bw, circular, ax, **shade_kwargs):
+def _violinplot(val, rug, side, shade, bw, circular, ax, **shade_kwargs):
     """Auxiliary function to plot violinplots."""
     if bw == "default":
         bw = "taylor" if circular else "experimental"
     x, density = kde(val, circular=circular, bw=bw)
 
-    if not rug:
+    if rug and side == "both":
+        side = "right"
+
+    if side == "left":
+        dens = -density
+    elif side == "right":
+        x = x[::-1]
+        dens = density[::-1]
+    elif side == "both":
         x = np.concatenate([x, x[::-1]])
-        density = np.concatenate([-density, density[::-1]])
+        dens = np.concatenate([-density, density[::-1]])
 
-    ax.fill_betweenx(x, density, alpha=shade, lw=0, **shade_kwargs)
+    ax.fill_betweenx(x, dens, alpha=shade, lw=0, **shade_kwargs)
     return density
 
 
-def cat_hist(val, rug, shade, ax, **shade_kwargs):
+def cat_hist(val, rug, side, shade, ax, **shade_kwargs):
     """Auxiliary function to plot discrete-violinplots."""
     bins = get_bins(val)
     _, binned_d, _ = histogram(val, bins=bins)
@@ -124,7 +133,15 @@ def cat_hist(val, rug, shade, ax, **shade_kwargs):
     heights = np.diff(bin_edges)
     centers = bin_edges[:-1] + heights.mean() / 2
 
-    left = None if rug else -0.5 * binned_d
+    if rug and side == "both":
+        side = "right"
+
+    if side == "right":
+        left = None
+    elif side == "left":
+        left = -binned_d
+    elif side == "both":
+        left = -0.5 * binned_d
 
     ax.barh(centers, binned_d, height=heights, left=left, alpha=shade, **shade_kwargs)
     return binned_d

arviz/plots/violinplot.py

@@ -15,6 +15,7 @@ def plot_violin(
     transform=None,
     quartiles=True,
     rug=False,
+    side="both",
     hdi_prob=None,
     shade=0.35,
     bw="default",
@@ -61,6 +62,10 @@ def plot_violin(
         intervals. Defaults to ``True``.
     rug: bool
         If ``True`` adds a jittered rugplot. Defaults to ``False``.
+    side : {"both", "left", "right"}, default "both"
+        If ``both``, both sides of the violin plot are rendered. If ``left`` or ``right``, only
+        the respective side is rendered. By separately plotting left and right halfs with
+        different data, split violin plots can be achieved.
     hdi_prob: float, optional
         Plots highest posterior density interval for chosen percentage of density.
         Defaults to 0.94.
@@ -162,6 +167,7 @@ def plot_violin(
         shade=shade,
         rug=rug,
         rug_kwargs=rug_kwargs,
+        side=side,
         bw=bw,
         textsize=textsize,
         labeller=labeller,
@@ -176,6 +182,9 @@ def plot_violin(
         backend = rcParams["plot.backend"]
     backend = backend.lower()
 
+    if side not in ("both", "left", "right"):
+        raise ValueError(f"'side' can only be 'both', 'left', or 'right', got: '{side}'")
+
     # TODO: Add backend kwargs
     plot = get_plotting_function("plot_violin", "violinplot", backend)
     ax = plot(**violinplot_kwargs)

arviz/tests/base_tests/test_plots_bokeh.py

@@ -848,8 +848,12 @@ def test_plot_parallel_exception(models, var_names):
 
 
 @pytest.mark.parametrize("var_names", (None, "mu", ["mu", "tau"]))
-def test_plot_violin(models, var_names):
-    axes = plot_violin(models.model_1, var_names=var_names, backend="bokeh", show=False)
+@pytest.mark.parametrize("side", ["both", "left", "right"])
+@pytest.mark.parametrize("rug", [True])
+def test_plot_violin(models, var_names, side, rug):
+    axes = plot_violin(
+        models.model_1, var_names=var_names, side=side, rug=rug, backend="bokeh", show=False
+    )
     assert axes.shape
 
 

arviz/tests/base_tests/test_plots_matplotlib.py

@@ -908,8 +908,10 @@ def test_plot_legend(models):
 
 
 @pytest.mark.parametrize("var_names", (None, "mu", ["mu", "tau"]))
-def test_plot_violin(models, var_names):
-    axes = plot_violin(models.model_1, var_names=var_names)
+@pytest.mark.parametrize("side", ["both", "left", "right"])
+@pytest.mark.parametrize("rug", [True])
+def test_plot_violin(models, var_names, side, rug):
+    axes = plot_violin(models.model_1, var_names=var_names, side=side, rug=rug)
     assert axes.shape
 
 

examples/bokeh/bokeh_plot_violin_single_sided.py

@@ -0,0 +1,23 @@
+"""
+Single sided Violinplot
+==========
+
+_thumb: .2, .8
+"""
+import arviz as az
+
+data = az.load_arviz_data("rugby")
+labeller = az.labels.MapLabeller(var_name_map={"defs": "atts | defs"})
+
+p1 = az.plot_violin(
+    data.posterior["atts"], side="left", backend="bokeh", show=False, labeller=labeller
+)
+p2 = az.plot_violin(
+    data.posterior["defs"],
+    side="right",
+    ax=p1,
+    backend="bokeh",
+    shade_kwargs={"color": "lightsalmon"},
+    show=True,
+    labeller=labeller,
+)

examples/matplotlib/mpl_plot_violin_single_sided.py

@@ -0,0 +1,14 @@
+"""
+Violin plot single sided
+===========
+
+_thumb: .2, .8
+_example_title: Single sided violin plot
+"""
+import arviz as az
+
+data = az.load_arviz_data("rugby")
+
+labeller = az.labels.MapLabeller(var_name_map={"defs": "atts | defs"})
+axs = az.plot_violin(data, var_names=["atts"], side="left", show=False)
+az.plot_violin(data, var_names=["defs"], side="right", labeller=labeller, ax=axs, show=True)