Fix kwargs in plot_graph

[yu-ta-sato]

Description

Fixed a plotting bug in plot_graph for homogeneous graphs when legend=True, legend_kwargs, and title are provided

Problem

plot_graph was forwarding legend, legend_kwargs, and title directly into lower-level GeoDataFrame.plot() kwargs. In this path:

• legend_kwargs leaked into edge plotting and triggered:
  • IndexError: index 1 is out of bounds for axis 0 with size 1
• title was also incorrectly passed through instead of being applied on the axes.

Root Cause

In _plot_homo_graph and style resolution, plot-control kwargs (legend, legend_kwargs, title) were not properly treated as city2graph-managed parameters. They were unintentionally propagated to GeoPandas/Matplotlib collection plotting calls.

Changes

• Marked legend, legend_kwargs, and title as reserved c2g keys so they are excluded from generic style kwargs.
• Updated homogeneous plotting flow to:
  • strip title, legend, legend_kwargs before edge/node plotting,
  • apply legend settings only on node plotting,
  • map legend_kwargs to GeoPandas’ expected legend_kwds,
  • set title via ax.set_title(...) after plotting.
• Improved plot parameter handling for column-driven styling by resolving pd.Series/column refs to numpy arrays where needed for consistent plotting behavior.

Validation

• Reproduced the originally reported failing snippet and confirmed it now runs successfully.

### Failed before

import geopandas as gpd
from shapely.geometry import Point, LineString
import matplotlib.pyplot as plt
import city2graph as c2g

nodes = gpd.GeoDataFrame(
    {"centrality_quantile": [1, 2]},
    geometry=[Point(0, 0), Point(1, 1)],
    crs="EPSG:4326",
)

edges = gpd.GeoDataFrame(
    {"frequency": [100, 200]},
    geometry=[LineString([(0, 0), (1, 1)]), LineString([(1, 1), (2, 2)])],
    crs="EPSG:4326",
)

fig, ax = plt.subplots(figsize=(5, 5))

c2g.plot_graph(
    nodes=nodes,
    edges=edges,
    node_color="centrality_quantile",
    edge_color="#bbbbbb",
    edge_linewidth=edges["frequency"] / 200,
    legend=True,
    legend_kwargs={"label": "Betweenness Centrality", "orientation": "horizontal"},
    title = "Central London Transit Network Betweenness Centrality of Stops",
    ax=ax,
)

plt.show()

### Works
# Removing `legend=True`, `legend_kwargs`, and `title`  a makes the same call succeed.

import geopandas as gpd
from shapely.geometry import Point, LineString
import matplotlib.pyplot as plt
import city2graph as c2g

nodes = gpd.GeoDataFrame(
    {"centrality_quantile": [1, 2]},
    geometry=[Point(0, 0), Point(1, 1)],
    crs="EPSG:4326",
)

edges = gpd.GeoDataFrame(
    {"frequency": [100, 200]},
    geometry=[LineString([(0, 0), (1, 1)]), LineString([(1, 1), (2, 2)])],
    crs="EPSG:4326",
)

fig, ax = plt.subplots(figsize=(5, 5))

c2g.plot_graph(
    nodes=nodes,
    edges=edges,
    node_color="centrality_quantile",
    edge_color="#bbbbbb",
    edge_linewidth=edges["frequency"] / 200,
    # legend=True,
    # legend_kwargs={"label": "Betweenness Centrality", "orientation": "horizontal"},
    # title = "Central London Transit Network Betweenness Centrality of Stops"
    ax=ax,
)

plt.show()

• Added a regression test covering the homogeneous case with node_color column, edge_linewidth series, legend=True, legend_kwargs, and title.

Related Issues

#121

Checklist

• I have read the Contributing Guide.
• I have updated the documentation, if necessary.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Pre-commit checks passed locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!