Add plotstyle: reusable KIT-branded matplotlib styling toolkit

Introduces a new plotstyle package providing consistent, presentation/thesis-ready
matplotlib figures: a KIT corporate-design color palette (categorical, sequential,
diverging, status), a theme applied via use() (KIT-black bottom spine only, left/bottom
ticks, horizontal gridlines, left-aligned titles, LaTeX text in Latin Modern Sans),
new_figure() with size presets and figure-level title/params subtitles, a
same-size colorbar() helper, style_legend() and panel_label() building blocks, and
savefig(). Ships as an optional "plotting" extra so core gallery installs stay
lightweight, with a full test suite and a runnable example notebook.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plotstyle/annotations.py

@@ -0,0 +1,97 @@
+"""Legend and panel-label helpers for consistent multi-panel figures."""
+
+from __future__ import annotations
+
+import warnings
+
+from matplotlib.axes import Axes
+from matplotlib.colors import to_rgba
+
+from .colors import INK
+
+_PANEL_LOCS = {
+    "upper left": (0.02, 0.98, "left", "top"),
+    "upper right": (0.98, 0.98, "right", "top"),
+    "lower left": (0.02, 0.08, "left", "bottom"),
+    "lower right": (0.98, 0.08, "right", "bottom"),
+}
+
+
+def style_legend(
+    ax: Axes,
+    loc: str = "outside right upper",
+    frameon: bool = False,
+    title: str = None,
+    **kwargs,
+):
+    """Add a figure-level legend, placed outside the axes by default.
+
+    Builds on `ax`'s handles/labels (or explicit `handles=`/`labels=` kwargs)
+    but attaches the legend to `ax`'s figure via `fig.legend(...)`, so it sits
+    outside the plot area rather than overlapping the data.
+
+    A `title` is strongly encouraged: an untitled legend floating outside the
+    axes loses its visual link to what it's describing, e.g.
+    `style_legend(ax, title="Series")`. Without one, this emits a warning
+    rather than failing — the legend still renders.
+    """
+    if title is None:
+        warnings.warn(
+            "style_legend() called without a title — an outside legend reads better with one, "
+            "e.g. style_legend(ax, title='Series').",
+            stacklevel=2,
+        )
+
+    fig = ax.get_figure()
+    handles = kwargs.pop("handles", None)
+    labels = kwargs.pop("labels", None)
+    if handles is None or labels is None:
+        handles, labels = ax.get_legend_handles_labels()
+
+    legend = fig.legend(handles, labels, loc=loc, frameon=frameon, title=title, **kwargs)
+    if legend.get_title() is not None:
+        legend.get_title().set_fontweight("bold")
+    return legend
+
+
+def panel_label(
+    ax: Axes,
+    label: str,
+    loc: str = "lower right",
+    fontweight: str = "bold",
+    box: bool = True,
+    **kwargs,
+):
+    """Add a panel label like "(a)" for multi-panel thesis/paper figures.
+
+    Defaults to the bottom-right corner, colored to match the axes' xlabel,
+    on a light-grey semi-transparent rounded box with a slim solid border.
+    Pass `box=False` for bare text with no box.
+    """
+    try:
+        x, y, ha, va = _PANEL_LOCS[loc]
+    except KeyError as exc:
+        raise ValueError(f"Unknown panel_label loc {loc!r}. Choose from {sorted(_PANEL_LOCS)}.") from exc
+
+    kwargs.setdefault("color", ax.xaxis.label.get_color())
+    if box:
+        kwargs.setdefault(
+            "bbox",
+            dict(
+                boxstyle="round,pad=0.3",
+                facecolor=to_rgba(INK["gridline"], alpha=0.8),
+                edgecolor=INK["baseline"],
+                linewidth=0.8,
+            ),
+        )
+
+    return ax.text(
+        x,
+        y,
+        f"({label})",
+        transform=ax.transAxes,
+        ha=ha,
+        va=va,
+        fontweight=fontweight,
+        **kwargs,
+    )