SHAP Prism

SHAP Prism visualizes an aligned matrix of precomputed SHAP values in two complementary views:

• plot_summary is a categorical-aware global beeswarm. Continuous features use a blue-purple-pink value scale by default; categorical features use discrete colors, markers, and feature-specific keys.
• plot_prism adds a shared-scale comparison of one focal feature across two to six declared subgroups.

Here, a multilevel categorical feature is a single-valued feature with three or more observed nonmissing levels. Binary categorical features use the same encoding.

Both functions describe one model output. They expect a finite two-dimensional SHAP matrix aligned row-for-row and column-for-column with the supplied feature table. The package does not fit models, compute SHAP values, choose subgroups, or perform causal or fairness inference.

Install

From a source checkout, install the package in editable mode:

python -m pip install -e .

Version 0.4.4 is verified on Python 3.12 with both the pinned publication stack and the latest dependency versions permitted by pyproject.toml. SHAP itself is not a runtime dependency because the plotting functions consume explanations that have already been computed. No empirical data or explanation table is bundled; deterministic synthetic values drive the quick start and tests.

The executed fully synthetic quick-start notebook generates its feature and contribution matrices in memory, demonstrates both plotting functions, and includes a ten-level categorical row rendered as seven raw levels plus pooled Others under the documented frequency-and-tie rule.

Quick start

Object, string, Boolean, and pandas categorical columns are detected automatically. Convert numeric category codes to a categorical dtype before plotting. Supply mixed numeric and categorical data as a pandas DataFrame with explicit column dtypes; a NumPy object array cannot preserve that distinction reliably and is rejected.

import json
from pathlib import Path

import numpy as np
import pandas as pd
from shap_prism import plot_prism, plot_summary

rng = np.random.default_rng(42)
n = 240
X = pd.DataFrame({
    "severity_band": pd.Categorical(
        np.tile(["low", "moderate", "high", "critical"], n // 4),
        categories=["low", "moderate", "high", "critical"],
        ordered=True,
    ),
    "device": np.tile(["A", "B", "C"], n // 3),
    "flag": np.tile([False, True], n // 2),
    "temperature": rng.normal(20, 4, n),
})

# Replace this illustrative matrix with values returned by your SHAP explainer.
phi = np.column_stack([
    X["severity_band"].cat.codes.to_numpy() - 1.5,
    np.select([X["device"].eq("A"), X["device"].eq("C")], [-0.5, 0.6], 0),
    np.where(X["flag"], 0.35, -0.35),
    0.08 * (X["temperature"] - 20),
])

summary = plot_summary(phi, X, output="categorical_summary")

site = np.repeat(["North", "Central", "South"], n // 3)
prism = plot_prism(
    phi,
    X,
    site,
    "severity_band",
    group_order=["North", "Central", "South"],
    output="severity_by_site",
)

Path("severity_by_site.json").write_text(
    json.dumps(prism.summary, indent=2),
    encoding="utf-8",
)

Each suffix-free output stem writes both PNG and PDF. An explicit .png or .pdf suffix writes only that format. The returned PrismResult exposes the Matplotlib figure, named axes, and a JSON-safe summary; the JSON file in the example is written explicitly.

Standalone category-key placement

plot_summary offers three placements without changing feature order, colors, markers, or SHAP values:

plot_summary(phi, X, category_key_placement="bottom")
plot_summary(phi, X, category_key_placement="right_stacked")

The bottom dock packs at most two adjacent feature blocks on one line when their measured labels fit. The right-stacked layout follows the displayed global feature order rather than the original DataFrame column order.

Color schemes and custom colors

The original Prism palette remains the default. The opt-in Okabe-Ito preset uses Okabe-Ito colors plus redundant marker shapes for categorical values and the perceptually ordered cividis map for continuous values:

plot_summary(phi, X, color_scheme="okabe_ito")
plot_prism(phi, X, site, "severity_band", color_scheme="okabe_ito")

Okabe-Ito is qualitative: in severity mode, the key order conveys severity; the hues identify categories. It is an accessibility-oriented option rather than a guarantee under every display or form of color-vision deficiency.

Custom categorical and continuous colors can be supplied independently:

plot_summary(
    phi,
    X,
    category_palette=["#22577A", "#38A3A5", "#C8553D", "#7B2CBF"],
    continuous_cmap=["#102A43", "#F6C85F", "#D1495B"],
)

plot_prism(
    phi,
    X,
    site,
    "severity_band",
    category_palette={
        "low": "#22577A",
        "moderate": "#38A3A5",
        "high": "#C8553D",
        "critical": "#7B2CBF",
    },
    global_category_palette=["#22577A", "#38A3A5", "#C8553D", "#7B2CBF"],
)

For plot_summary, the custom categorical sequence restarts for each feature after its category order has been resolved. In plot_prism, category_palette controls the focal feature and global_category_palette controls other categorical rows. Explicit channel overrides take precedence over color_scheme. Missing values remain a gray X. When a categorical feature has more than eight observed nonmissing levels, the same eight-entry encoding is used for seven retained levels plus a final Others level.

Category order and color

category_order_mode supports three display rules:

For an unpooled categorical feature in severity mode under the default Prism scheme, severity_direction="higher" places categories associated with lower model output at the blue end and higher output at the pink end. Under Okabe-Ito or custom colors, the ordered key rather than a hue progression carries this meaning. Use "lower" when lower output is the more severe direction. For a categorical focal feature in plot_prism, category_order can instead provide the complete exact order. An explicit order replaces the selected ordering mode; above eight levels, however, it controls display order and frequency-tie resolution rather than overriding frequency-based retention. The returned summary records the source and resolved order used in the figure.

The built-in palettes support eight nonmissing display entries. Marker shapes provide a redundant cue. If more than eight raw levels are observed, SHAP Prism retains seven raw levels by decreasing full-data frequency, breaking ties by the resolved category order, and pools every remaining row into a final Others entry. Retained levels keep their resolved display order; Others is always last. A literal raw category named Others, if present, is always absorbed into that pooled entry so that the key remains unambiguous. This rule uses all aligned rows rather than the display sample and applies to plot_summary, the global panel of plot_prism, and a categorical focal feature in plot_prism.

Color position never encodes numerical distance between category means; in data and alphabetical modes, colors are identifiers only. In severity mode, the retained levels follow category-level mean signed SHAP order, while the forced-final pooled Others color is an identifier rather than a severity endpoint. Missing values use a gray X. When a literal raw category named Missing coexists with actual missing cells, the raw category keeps its categorical encoding and the gray-X entry is labeled Missing (NA) so the legend and returned summary remain unambiguous.

Outputs and reproducibility

Feature ranking, axis limits, category means, subgroup counts, and optional subgroup means use all aligned rows. max_points changes only the displayed dots. Sampling and vertical packing are deterministic for a fixed random_state.

The summary records, among other fields:

• global feature order and the displayed rows;
• category levels, colors, markers, and category-level mean signed SHAP values;
• retained and collapsed high-cardinality levels and their all-row counts;
• the resolved missing-value label when actual missing cells are present;
• the source and resolved form of each categorical order;
• subgroup order, full group counts, means, and shared SHAP limits;
• display-sampling settings and written output paths.

This reporting contract makes it possible to inspect a figure without extracting values from rendered pixels.

Supported inputs and limits

• shap_values and features must have identical two-dimensional shapes with at least two rows and one feature.
• Array inputs require feature_names. DataFrame and Series indices are checked when both sides provide them.
• plot_prism requires two to six nonmissing groups and one focal feature.
• max_features and max_global_features accept 1 to 15; max_points must be at least 20.
• Categorical features use at most eight nonmissing display entries: seven separately encoded levels plus Others when more than eight raw levels are observed.

The detailed parameter, return, and error contracts live in the public function docstrings:

help(plot_summary)
help(plot_prism)

Interpretation

Horizontal position is the SHAP contribution on the declared model-output scale. Color or marker identifies the observed feature value, not subgroup membership. In plot_prism, compare groups only along their common horizontal scale; vertical spread is point packing, not a probability-density axis. The hollow diamond, when enabled, is the signed all-row subgroup mean.

Differences between groups can reflect model interactions, feature composition, dependence, data quality, model error, or chance. SHAP Prism displays these patterns. Study design and statistical inference remain separate.

Examples and verification

python examples/continuous_demo.py
python examples/categorical_demo.py
python examples/color_schemes_demo.py
python -m unittest discover -s tests -v
python scripts/render_visual_gallery.py --output visual_checks

The optional Palmer Penguins upstream builder requires user-acquired data kept outside the repository and the reproduction extra:

python -m pip install ".[reproduction]"
python examples/palmer_penguins_global_categories.py --help

See ARTICLE_DATA_ACQUISITION.md for official source links, external schemas, and the no-redistribution boundary. The repository does not claim one-command empirical article reproduction.

The automated suite covers 30 to 2,000 rows, 3 to 20 features, two to six groups, unequal groups, missing values, ordinary and high-cardinality categorical features, every ordering mode, deterministic sampling, and journal-size PNG/PDF output.

The repository also contains:

• DEVELOPMENT.md: local development and verification;
• COMPATIBILITY.md: verified environment;
• DATA_LICENSES.md: the data-free release boundary;
• ARTICLE_DATA_ACQUISITION.md: external article inputs and official source locations;
• LICENSE_SCOPE.md: exact software, data, and third-party licensing boundaries.

Encoded columns

The package plots one supplied attribution column per displayed feature. If several encoded columns represent one source feature, aggregate them before plotting and verify that local additivity is preserved:

encoded_columns = [2, 3, 4]
concept_phi = shap_values[:, encoded_columns].sum(axis=1)
remaining = np.delete(shap_values, encoded_columns, axis=1)
display_phi = np.column_stack([remaining, concept_phi])

np.testing.assert_allclose(
    display_phi.sum(axis=1),
    shap_values.sum(axis=1),
)

This sum is a grouped encoded contribution, not automatically the Shapley or Owen value of the unencoded feature. Record the column mapping.

Citation and license

Use CITATION.cff for the current software citation. The original Python source, examples, tests, and project documentation are MIT-licensed, with copyright held jointly by József Pintér and Servando Sibón Muñoz. Third-party and data boundaries are stated in LICENSE_SCOPE.md and THIRD_PARTY_NOTICES.md.