Dev Create Diagram

Create a diagram or visualization for a forge-gpu lesson using the project's matplotlib diagram infrastructure. Diagrams increase reader engagement and help learners understand the topics being taught.

When to use this skill:

• A lesson would benefit from a visual explanation of a concept
• You need to show geometric relationships, data flow, memory layout, or mathematical concepts visually
• A concept is hard to explain with text alone — a diagram makes it click
• You're writing a lesson with /dev-math-lesson, /dev-engine-lesson, or /dev-gpu-lesson and want to add visual aids

When NOT to use this skill:

• The concept is a sequential flow better suited to a Mermaid diagram
• The concept is a formula better expressed with KaTeX math notation
• A screenshot of the running program already shows the concept clearly

Arguments

The user (or you) provides:

• Lesson key: e.g. math/01, gpu/04, engine/04
• Diagram name: e.g. vector_addition.png, stack_vs_heap.png
• Description: what the diagram should show

If any are missing, infer from context or ask.

Steps

1. Determine the diagram module

Diagram functions are organized by track and lesson number under scripts/forge_diagrams/<track>/lesson_NN.py:

• scripts/forge_diagrams/math/lesson_NN.py — math lessons
• scripts/forge_diagrams/gpu/lesson_NN.py — GPU lessons
• scripts/forge_diagrams/engine/lesson_NN.py — engine lessons
• scripts/forge_diagrams/ui/lesson_NN.py — UI lessons
• scripts/forge_diagrams/assets/lesson_NN.py — asset pipeline lessons
• scripts/forge_diagrams/physics/lesson_NN.py — physics lessons

If a lesson_NN.py file does not exist yet for the lesson, create one.

2. Write the diagram function

Add a new function to the appropriate per-lesson module following the existing patterns.

Required imports and helpers:

import matplotlib.patheffects as pe
import matplotlib.pyplot as plt
import numpy as np

from .._common import FORGE_CMAP, STYLE, draw_vector, save, setup_axes

pe is needed for the text readability stroke (path_effects=[pe.withStroke(...)]) used throughout diagram code. FORGE_CMAP is a custom colormap for heatmaps and gradient visualizations — import it when using imshow() or pcolormesh().

Theme colors — always use STYLE dict values, never hardcoded colors:

Function template:

# ---------------------------------------------------------------------------
# category/NN-lesson-name — diagram_name.png
# ---------------------------------------------------------------------------


def diagram_diagram_name():
    """Brief description of what the diagram shows."""
    fig = plt.figure(figsize=(W, H), facecolor=STYLE["bg"])
    ax = fig.add_subplot(111)
    setup_axes(ax, xlim=(...), ylim=(...))

    # --- Draw content here ---

    # Title with vertical padding (pad >= 12 to avoid crowding content)
    ax.set_title(
        "Diagram Title",
        color=STYLE["text"],
        fontsize=14,
        fontweight="bold",
        pad=12,
    )

    fig.tight_layout()
    save(fig, "category/NN-lesson-name", "diagram_name.png")

Common figure sizes:

• Vector/math diagrams: (7, 7) — square
• Comparison (side-by-side): (10, 5) — landscape
• Memory/system layouts: (10, 8) — tall
• General concepts: (8, 7) to (10, 7) — balanced

3. Quality checks before saving

After writing the diagram function, verify these requirements:

No overlapping labels

Labels must not overlap each other or be drawn on top of lines/arrows. To prevent overlap:

• Use label_offset parameter in draw_vector() to shift labels away from arrows and other text
• For manually placed ax.text() calls, compute positions that avoid other text elements
• When labels are dense, reduce fontsize or increase figure dimensions
• Test with the actual data — positions that look fine in pseudocode may overlap when rendered at the final DPI
• Use ha (horizontal alignment) and va (vertical alignment) parameters to anchor text away from crowded areas
• If two labels would land in the same region, offset one of them and optionally add a thin leader line connecting the label to its element

No lines drawn across labels

Lines, arrows, and grid elements must not pass through text:

• Draw text elements with zorder=5 or higher so they render above lines
• Use path_effects=[pe.withStroke(linewidth=3, foreground=STYLE["bg"])] on all text to create a background halo that visually separates text from crossing lines
• Where possible, route lines around label regions rather than through them
• For grid lines that pass behind text, the stroke effect provides sufficient clearance

Title-to-content vertical padding

There must be visible vertical space between the title and the topmost content element (data points, arrows, labels, boxes). Crowded titles make diagrams feel cramped and reduce readability.

• Use pad=12 or greater in ax.set_title() — this adds points of space between the title baseline and the axes top edge
• If using fig.suptitle() instead, set y=0.97 or lower and ensure the subplot top margin accommodates it (via fig.subplots_adjust(top=0.90) or fig.tight_layout(rect=[0, 0, 1, 0.95]))
• After generating the diagram, visually confirm that the title does not touch or crowd the top of the content area

Use theme colors exclusively

Every color in the diagram must come from the STYLE dictionary:

• Never use hardcoded color strings like "red", "blue", "#ff0000"
• Never use matplotlib default colors
• Use accent colors semantically: accent1 for the primary subject, accent2 for secondary/comparison, accent3 for tertiary/reference, accent4 for special highlights
• Use text for primary labels and text_dim for secondary annotations
• Use surface for filled regions and grid for subtle structural elements

4. Register the diagram

Wire up the function so the CLI can find it. Three files need changes:

1. Per-lesson file — already done in step 2
2. Track __init__.py — re-export the function
3. __main__.py — import and register in DIAGRAMS dict

Example for a new math lesson (math/NN):

# 1. Function is in scripts/forge_diagrams/math/lesson_NN.py (already written)

# 2. In scripts/forge_diagrams/math/__init__.py — add to __all__ and imports:
from .lesson_NN import diagram_new_concept
# ... and add "diagram_new_concept" to the __all__ list

# 3. In scripts/forge_diagrams/__main__.py:
# Import section:
from .math import diagram_new_concept

# DIAGRAMS dict:
"math/NN": [
    ("new_concept.png", diagram_new_concept),
],

# LESSON_NAMES:
"math/NN": "math/NN-concept-name",

5. Generate the diagram

Run the diagram generator to produce the PNG:

python scripts/forge_diagrams --lesson category/NN

The output goes to lessons/category/NN-name/assets/diagram_name.png at 200 DPI.

6. Verify the output

After generating, inspect the diagram for:

• No overlapping labels — all text is readable and distinct
• No lines across labels — text has clear background separation
• Title padding — visible gap between title and content below it
• Theme colors — dark background, no default matplotlib colors visible
• Readability — text is large enough (>= 9pt), contrast is sufficient
• Correct content — the diagram accurately represents the concept

7. Reference in the README

Add the diagram to the lesson's README.md:

![Description of what the diagram shows](assets/diagram_name.png)

Place diagrams near the text that explains the concept they illustrate — before the detailed explanation, not after it. Seeing the visual first helps the reader build intuition before reading the technical details.

8. Run Python linting

Verify the new code passes linting:

ruff check scripts/forge_diagrams/
ruff format --check scripts/forge_diagrams/

Auto-fix if needed:

ruff check --fix scripts/forge_diagrams/
ruff format scripts/forge_diagrams/

Shared helpers reference

setup_axes(ax, xlim=None, ylim=None, grid=True, aspect="equal")

Applies the dark theme to axes: background color, grid styling, tick colors, spine colors. Call this on every axes object before drawing.

draw_vector(ax, origin, vec, color, label=None, label_offset=(0.15, 0.15), lw=2.5)

Draws a labeled arrow from origin to origin + vec. The label gets a background stroke for readability. Use label_offset to prevent overlap with nearby elements.

save(fig, lesson_path, filename)

Saves the figure to lessons/{lesson_path}/assets/{filename} at 200 DPI with the dark background. Creates the assets directory if needed. Always call this as the last step — it also closes the figure.

FORGE_CMAP

A custom 4-color colormap (bg -> accent1 -> accent2 -> warn) for heatmaps and gradient visualizations. Use with imshow(), pcolormesh(), etc.

Common patterns

Text with readability stroke

All text over diagram content should use a background stroke:

ax.text(
    x, y, "Label",
    color=STYLE["accent1"],
    fontsize=11,
    fontweight="bold",
    ha="center", va="center",
    path_effects=[pe.withStroke(linewidth=3, foreground=STYLE["bg"])],
)

Legend styling

leg = ax.legend(loc="upper right", fontsize=10, framealpha=0.3,
                edgecolor=STYLE["grid"])
for text in leg.get_texts():
    text.set_color(STYLE["text"])

Multi-panel figures

For side-by-side comparisons, use subplots and apply setup_axes to each:

fig = plt.figure(figsize=(10, 5), facecolor=STYLE["bg"])
ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)
setup_axes(ax1, ...)
setup_axes(ax2, ...)

Filled regions

from matplotlib.patches import Polygon, Rectangle, FancyBboxPatch
# Use STYLE["surface"] for fills, STYLE["accent*"] for borders

Post-creation validation (MANDATORY)

After writing the diagram function, before committing:

1. Read the lesson README section that references this diagram
2. Verify every visual element matches the README's description:
   • If README says "oriented bounding box", the diagram must rotate the box
   • If README shows equation y = f(x), the diagram must plot that exact function
   • If README describes 3 pipeline stages, the diagram must show all 3
3. Run the diagram and visually inspect the output
4. Verify the docstring matches what was actually plotted (not copy-pasted from a different diagram)

For a thorough cross-check, invoke /dev-review-diagrams with the lesson key.

Common mistakes

• Hardcoded colors — Using "red" or "#ff0000" instead of STYLE["accent2"]. Every color must come from the theme.
• Missing stroke on text — Text without path_effects becomes unreadable when grid lines or other elements pass behind it.
• Crowded title — Using pad=0 or omitting pad in set_title(), causing the title to sit directly on top of the data.
• Overlapping labels — Placing two labels at similar coordinates without adjusting offsets. Always check label positions against each other.
• Wrong module — Adding a math diagram to a GPU lesson file or vice versa. Match the track directory and lesson number to the lesson category.
• Forgetting to register — Writing the function but not adding it to the DIAGRAMS dict in __main__.py. The CLI won't find unregistered diagrams.
• Forgetting to re-export — Adding the function to lesson_NN.py but not adding it to the track's __init__.py __all__ list and imports.
• Forgetting the import — Adding the entry to DIAGRAMS but not importing the function at the top of __main__.py.
• Not running linting — The diagram scripts must pass ruff check and ruff format before committing.