Add quantile line aesthetics to geom_violin

Adds quantile_color, quantile_size, and quantile_linetype parameters so
the quantile lines drawn by draw_quantiles can be styled independently
of the violin outline.

closes #1050

Author: has2k1

plotnine/geoms/geom_violin.py

@@ -45,6 +45,12 @@ class geom_violin(geom):
         'left-right'  # Alternate (left first) half violins by the group
         'right-left'  # Alternate (right first) half violins by the group
         ```
+    quantile_color : str | tuple, default=None
+        Color of the quantile lines.
+    quantile_size : str, default="o"
+        The linewidth of the quantile lines.
+    quantile_linetype : float, default=1.5
+        The linetype of the quantile lines.
 
     See Also
     --------
@@ -64,6 +70,10 @@ class geom_violin(geom):
         "stat": "ydensity",
         "position": "dodge",
         "draw_quantiles": None,
+        "quantile_color": None,
+        "quantile_colour": None,
+        "quantile_size": None,
+        "quantile_linetype": None,
         "style": "full",
         "scale": "area",
         "trim": True,
@@ -175,6 +185,8 @@ def draw_panel(
             )
 
             if quantiles is not None:
+                from plotnine.mapping._atomic import broadcast_ae_value
+
                 # Get dataframe with quantile segments and that
                 # with aesthetics then put them together
                 # Each quantile segment is defined by 2 points and
@@ -187,6 +199,19 @@ def draw_panel(
                     [make_quantile_df(df, quantiles), aes_df], axis=1
                 )
 
+                color = params["quantile_colour"] or params["quantile_color"]
+                n = len(segment_df)
+                if color:
+                    segment_df["color"] = broadcast_ae_value(color, "color", n)
+                if linetype := params["quantile_linetype"]:
+                    segment_df["linetype"] = broadcast_ae_value(
+                        linetype,
+                        "linetype",
+                        n,
+                    )
+                if size := params["quantile_size"]:
+                    segment_df["size"] = size
+
                 # plot quantile segments
                 geom_path.draw_group(
                     segment_df,

tests/baseline_images/test_geom_violin/quantile_aesthetics.png

@@ -140,3 +140,15 @@ def test_overlap():
     )
 
     assert p == "overlap"
+
+
+def test_quantile_aesthetics():
+    p = ggplot(data, aes("x")) + geom_violin(
+        aes(y="y"),
+        size=0.5,
+        draw_quantiles=[0.25, 0.5, 0.75],
+        quantile_size=1,
+        quantile_color=(0.9, 0.2, 0.8),
+        quantile_linetype=(0, (4, 4, 1, 4)),
+    )
+    assert p == "quantile_aesthetics"