Borders between similar colours

In a choropleth, two neighbours of similar colour (both pale, or both dark) merge into one patch when the outline is close to them in tone: the border between them disappears. border_contrast finds those shared borders and redraws only them, in a colour that stands out; the polygons keep their own outline.

The same map is drawn three times with a pale outline (stroke="#eeeeee") on a blue palette:

• without the option, where the pale departments run together;

• with border_contrast="check", which reveals the weak borders in magenta (and warns with their number);

• with border_contrast=True, which redraws them dark.

Data: mortality from heart failure, rate per 100,000 inhabitants (Sante publique France, ODISSE).

import warnings
from pathlib import Path

import pandas as pd

import mappyng as mp
from mappyng.data import load_france_departments

deps = load_france_departments().to_crs(2154)
deps = deps[deps["COD_GEO"].str.len() == 2]    # metropolitan France
df = pd.read_csv(Path("data") / "cardio_mortalite_dep.csv", dtype={"code_dep": str})
deps = deps.merge(df, left_on="COD_GEO", right_on="code_dep", how="left")


def build(title, **options):
    return (
        mp.Map(deps, width=620, height="auto", border_radius=0)
        + mp.Choropleth(
            deps, column="taux_mortalite", cmap="Blues", method="Quantiles",
            num_classes=5, stroke="#eeeeee", stroke_width=0.6,
            legend=mp.Legend("Deaths / 100,000", orientation="horizontal",
                             decimals=0, swatch_width=18, fontsize=7,
                             label_rotation=0, nodata_label="No data"),
            **options,
        )
        + mp.Title(title)
    )


# Without the option: pale neighbours run together.
m_off = build("Without border_contrast")

# Check: the weak borders in magenta. The warning that counts them is silenced
# here only to keep the example output short.
with warnings.catch_warnings():
    warnings.simplefilter("ignore")
    m_check = build("border_contrast='check'", border_contrast="check")

# Fix: only those shared borders are redrawn, dark on pale areas.
m_fix = build("border_contrast=True", border_contrast=True)