fix(themes): adapt linespacing across the matplotlib 3.11 boundary

The 3.11 text overhaul changes how line height is derived from the
font, so the previous numeric linespacing values no longer render
consistently. Select per-version linespacing ("normal" on >=3.11,
the prior numeric values otherwise) in theme_gray, theme_matplotlib,
theme_seaborn, and theme_void.

Also add a CI step that downgrades matplotlib below 3.11 so the lower
bound stays tested.

Author: has2k1

.github/workflows/testing.yml

@@ -42,6 +42,7 @@ jobs:
           # Install as an editable so that the coverage path
           # is predicable
           uv run uv pip install --resolution=${{ matrix.resolution }} -e ".[extra,test]"
+          uv lock --upgrade-package "matplotlib<3.11.0"
 
       - name: Environment Information
         run: uv pip list

plotnine/themes/theme_gray.py

@@ -27,6 +27,12 @@ class theme_gray(theme):
     """
 
     def __init__(self, base_size=11, base_family=None):
+        import matplotlib as mpl
+        from packaging import version
+
+        MPL311 = version.parse(mpl.__version__) >= version.parse("3.11.0")
+        ls1, ls2 = ("normal", None) if MPL311 else (0.9, 1.5)
+
         base_family = base_family or get_option("base_family")
         half_line = base_size / 2
         quarter_line = base_size / 4
@@ -47,7 +53,7 @@ def __init__(self, base_size=11, base_family=None):
                 color="black",
                 ma="center",
                 size=base_size,
-                linespacing=0.9,
+                linespacing=ls1,  # pyright: ignore[reportArgumentType]
                 rotation=0,
                 margin=margin(),
             ),
@@ -137,7 +143,7 @@ def __init__(self, base_size=11, base_family=None):
             strip_text=element_text(
                 color="#1A1A1A",
                 size=base_size * 0.8,
-                linespacing=1.5,
+                linespacing=ls2,
                 margin=margin_auto(half_line * 0.8),
             ),
             strip_text_y=element_text(rotation=-90),

plotnine/themes/theme_matplotlib.py

@@ -30,6 +30,10 @@ class theme_matplotlib(theme):
 
     def __init__(self, rc=None, fname=None, use_defaults=True):
         import matplotlib as mpl
+        from packaging import version
+
+        MPL311 = version.parse(mpl.__version__) >= version.parse("3.11.0")
+        ls1, ls2 = ("normal", None) if MPL311 else (1, 1.5)
 
         m = get_option("base_margin")
         base_size = mpl.rcParams.get("font.size", 11)
@@ -41,7 +45,7 @@ def __init__(self, rc=None, fname=None, use_defaults=True):
             rect=element_rect(size=linewidth),
             text=element_text(
                 size=base_size,
-                linespacing=1,
+                linespacing=ls1,  # pyright: ignore[reportArgumentType]
                 rotation=0,
                 margin={},
             ),
@@ -111,7 +115,7 @@ def __init__(self, rc=None, fname=None, use_defaults=True):
                 fill="#D9D9D9", color="black", size=linewidth
             ),
             strip_text=element_text(
-                linespacing=1.5,
+                linespacing=ls2,
                 margin=margin_auto(half_line * 0.8),
             ),
             strip_text_y=element_text(rotation=-90),

plotnine/themes/theme_seaborn.py

@@ -39,8 +39,14 @@ def __init__(
         font="sans-serif",
         font_scale=1,
     ):
+        import matplotlib as mpl
+        from packaging import version
+
         from .seaborn_rcmod import set_theme
 
+        MPL311 = version.parse(mpl.__version__) >= version.parse("3.11.0")
+        _, ls2 = ("normal", None) if MPL311 else (1, 1.5)
+
         rcparams = set_theme(
             context=context, style=style, font=font, font_scale=font_scale
         )
@@ -126,7 +132,7 @@ def __init__(
             strip_background=element_rect(color="none", fill="#D1CDDF"),
             strip_text=element_text(
                 size=base_size * 0.8,
-                linespacing=1.5,
+                linespacing=ls2,
                 margin=margin_auto(half_line * 0.8),
             ),
             strip_text_y=element_text(rotation=-90),

plotnine/themes/theme_void.py

@@ -24,6 +24,12 @@ class theme_void(theme):
     """
 
     def __init__(self, base_size=11, base_family=None):
+        import matplotlib as mpl
+        from packaging import version
+
+        MPL311 = version.parse(mpl.__version__) >= version.parse("3.11.0")
+        ls1, _ = ("normal", None) if MPL311 else (0.9, 1.5)
+
         base_family = base_family or get_option("base_family")
         m = get_option("base_margin")
         # Use only inherited elements and make everything blank
@@ -36,7 +42,7 @@ def __init__(self, base_size=11, base_family=None):
                 style="normal",
                 color="black",
                 size=base_size,
-                linespacing=0.9,
+                linespacing=ls1,
                 rotation=0,
                 margin=margin(),
             ),