Pad stacked colorbars past the previous one's measured extent (#687)

The fixed stacking pad cleared an inner colorbar's tick labels but not its axis
label (e.g. a rotated "instance_id"), so a longer label still overlapped the next
colorbar — worst for vertical (left/right) colorbars. Instead, draw each colorbar
and measure how far its ticks/labels/axis-label extend beyond its box on the outer
side, then pad the next stacked colorbar past that (plus a small gap). Replaces the
fixed CBAR_STACK_PAD_INCHES with the measured extent + CBAR_STACK_GAP_INCHES.

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -19,6 +19,7 @@
 from dask.dataframe import DataFrame as DaskDataFrame
 from geopandas import GeoDataFrame
 from matplotlib.axes import Axes
+from matplotlib.backend_bases import RendererBase
 from matplotlib.colors import Colormap, LogNorm, Normalize
 from matplotlib.figure import Figure
 from mpl_toolkits.axes_grid1 import make_axes_locatable
@@ -44,7 +45,7 @@
     CBAR_DEFAULT_FRACTION,
     CBAR_DEFAULT_LOCATION,
     CBAR_DEFAULT_PAD,
-    CBAR_STACK_PAD_INCHES,
+    CBAR_STACK_GAP_INCHES,
     ChannelLegendEntry,
     CmapParams,
     ColorbarSpec,
@@ -1532,6 +1533,9 @@ def _draw_colorbar(
             side_counts: dict[str, int],
             clearance: dict[str, float],
             axes_size_in: tuple[float, float],
+            prev_outer: dict[str, float],
+            renderer: RendererBase,
+            dpi: float,
         ) -> None:
             norm = spec.mappable.norm
             if isinstance(norm, LogNorm):
@@ -1566,19 +1570,18 @@ def _draw_colorbar(
 
             # Append the colorbar axes through the panel's shared divider. This steals space from the
             # panel (so the colorbar matches the equal-aspect plot's extent) and keeps it inside the
-            # panel's grid cell, never overflowing into a neighbouring panel. The first colorbar on a
-            # side is padded past the panel's own decorations (ticks/labels/title) plus the requested
-            # pad; each subsequent one uses an absolute pad wide enough to clear the previous
-            # colorbar's tick labels instead of overlapping them.
-            # Sides with no decorations to clear (typically "right") keep the relative pad, which
-            # matches the historical placement; sides whose ticks/labels/title must be cleared
-            # (left/top/bottom), and stacked colorbars, use an absolute pad in inches.
+            # panel's grid cell, never overflowing into a neighbouring panel.
+            # Pad of the first colorbar on a side clears the panel's own decorations (ticks/labels/
+            # title); a side with negligible clearance (typically "right") keeps the relative pad to
+            # match the historical placement. Each subsequent (stacked) colorbar is padded past the
+            # *measured* outer extent of the previous one (its tick labels AND axis label) so it never
+            # overlaps it.
             n_on_side = side_counts.get(location, 0)
             ref_in = axes_size_in[0] if location in {"left", "right"} else axes_size_in[1]
             side_clearance = clearance.get(location, 0.0)
             pad_spec: str | Fixed
             if n_on_side:
-                pad_spec = Fixed(CBAR_STACK_PAD_INCHES)
+                pad_spec = Fixed(prev_outer.get(location, 0.0) + CBAR_STACK_GAP_INCHES)
             elif side_clearance > _CBAR_DECORATION_EPS_INCHES:
                 pad_spec = Fixed(side_clearance + max(pad, 0.0) * ref_in)
             else:
@@ -1615,6 +1618,21 @@ def _draw_colorbar(
                 with contextlib.suppress(Exception):
                     cb.solids.set_alpha(spec.alpha)
 
+            # Measure how far this colorbar's own ticks/labels/axis-label extend beyond its box on
+            # the outer side, so the next stacked colorbar on this side can be padded clear of them.
+            fig.canvas.draw()
+            cb_box = cax.get_window_extent(renderer)
+            cb_tight = cax.get_tightbbox(renderer)
+            if location == "right":
+                outer = (cb_tight.x1 - cb_box.x1) / dpi
+            elif location == "left":
+                outer = (cb_box.x0 - cb_tight.x0) / dpi
+            elif location == "top":
+                outer = (cb_tight.y1 - cb_box.y1) / dpi
+            else:  # bottom
+                outer = (cb_box.y0 - cb_tight.y0) / dpi
+            prev_outer[location] = max(0.0, outer)
+
         # go through tree
 
         for i, cs in enumerate(coordinate_systems):
@@ -1829,8 +1847,9 @@ def _draw_colorbar(
                 # One divider per panel so multiple colorbars on the same panel stack via the divider.
                 divider = make_axes_locatable(axis)
                 side_counts: dict[str, int] = {}
+                prev_outer: dict[str, float] = {}
                 for spec in unique_specs:
-                    _draw_colorbar(spec, fig, divider, side_counts, clearance, axes_size_in)
+                    _draw_colorbar(spec, fig, divider, side_counts, clearance, axes_size_in, prev_outer, renderer, dpi)
 
         if fig_params.fig is not None and save is not None:
             save_fig(fig_params.fig, path=save)

src/spatialdata_plot/pl/render_params.py

@@ -219,9 +219,9 @@ class ChannelLegendEntry:
 CBAR_DEFAULT_LOCATION = "right"
 CBAR_DEFAULT_FRACTION = 0.075
 CBAR_DEFAULT_PAD = 0.015
-# Absolute pad (inches) between stacked colorbars on the same side, wide enough to clear the
-# inner colorbar's tick labels so they don't overlap the next colorbar.
-CBAR_STACK_PAD_INCHES = 0.5
+# Small gap (inches) added beyond a stacked colorbar's measured outer extent (its tick labels and
+# axis label) before placing the next colorbar on the same side, so they never overlap.
+CBAR_STACK_GAP_INCHES = 0.08
 
 
 @dataclass