Feature: Sankey diagrams

[cvanelteren]

Add layered Sankey diagram support with flow-aware label placement, expose the layered Sankey API with rc defaults and tests, and include a new gallery example in docs/examples/plot_types to showcase publication-ready Sankey styling and labels.

Closes #457

import ultraplot as uplt

fig, ax = uplt.subplots(figsize=(5, 2))
nodes = [
    "Budget",
    "Ops",
    "R&D",
    "Marketing",
    "Infra",
    "People",
    "Product",
    "Campaigns",
    "Savings",
]
flows = [
    dict(source="Budget", target="Ops", value=5, label="Ops", group="ops"),
    dict(source="Budget", target="R&D", value=3, label="R&D", group="rnd"),
    dict(source="Budget", target="Marketing", value=2, label="Mkt", group="mkt"),
    dict(source="Ops", target="Infra", value=2.5, label="Infra", group="ops"),
    dict(source="Ops", target="People", value=2.0, label="People", group="ops"),
    dict(source="R&D", target="Product", value=2.2, label="Prod", group="rnd"),
    dict(source="R&D", target="Savings", value=0.8, label="Save", group="rnd"),
    dict(source="Marketing", target="Campaigns", value=1.7, label="Camps", group="mkt"),
    dict(source="Marketing", target="Savings", value=0.2, label="Save", group="mkt"),
]

ax.sankey(
    nodes=nodes,
    flows=flows,
    style="budget",
    node_label_box=True,  # default rounded white box
    node_label_kw=dict(fontsize=9, color="0.15"),
    flow_labels=True,
    value_format="{:.1f}",
    group_cycle=["#f2a65a", "#7fb069", "#3c91e6"],
    flow_other=0.25,
    other_label="Other",
    node_label_outside="auto",
)
uplt.show(block=1)

[codecov]

Codecov Report

❌ Patch coverage is 86.09865% with 93 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/axes/plot_types/sankey.py	84.39%	41 Missing and 23 partials ⚠️
ultraplot/axes/plot.py	57.35%	14 Missing and 15 partials ⚠️

📢 Thoughts on this report? Let us know!

[beckermr]

I see defaults not in the rc config. Generally we should add everything there. Otherwise, LGTM.