Feature: Implement remaining diagram types (19 of 23 unimplemented)

[anand-testcompare]

Summary

The schema defines 23 diagram types, but only 4 have actual distinct implementations. The remaining 19 types fall back to the flowchart template.

Current State

Implemented with distinct templates (4):

Type	Implementation
flowchart	Default rectangles, straight arrows, TB layout
architecture	Wider spacing, elbow connectors
mindmap	Radial layout via applyRadialLayout()
sequence	Broken - see #68

Unimplemented (19) - all fall back to flowchart:

orgchart, class, er, gantt, timeline, tree, network, 
dataflow, state, swimlane, concept, fishbone, swot, 
pyramid, funnel, venn, matrix, infographic, decision-tree

Fallback logic in packages/backend/lib/templates/index.ts:

return TEMPLATES[diagramType.toLowerCase()] ?? FlowchartTemplate;

Scope

Each diagram type needs:

1. Template - Node shapes, colors, spacing defaults
2. Layout algorithm - Positioning logic (dagre config or custom)
3. Grading criteria - Visual evaluation prompts
4. Prompts - Generation guidance for the AI

Needs Grooming

This issue needs to be groomed and split into logical phases based on priority. Suggested approach:

Phase 1: High-value, achievable

Types that can be implemented with dagre config tweaks + shape mappings:

• decision-tree (already has layout config, needs template)
• orgchart (hierarchical TB layout)
• tree (similar to orgchart)
• state (already has layout config)

Phase 2: Moderate complexity

Types needing custom layout logic:

• er (entity-relationship with crow's foot notation)
• class (UML class diagrams with compartments)
• swimlane (lane-based horizontal partitioning)
• dataflow (DFD notation)

Phase 3: Specialized/Complex

Types requiring significant custom rendering:

• gantt (timeline bars, dependencies)
• timeline (chronological axis)
• network (force-directed or hierarchical)
• fishbone (Ishikawa cause-effect)

Phase 4: Evaluate necessity

Types that may not be worth implementing or could be consolidated:

• concept (may overlap with mindmap)
• swot (2x2 matrix - simple)
• pyramid / funnel (stacked shapes)
• venn (overlapping circles)
• matrix (grid layout)
• infographic (too generic?)

Next Steps

1. Groom this issue into separate phase issues
2. Prioritize based on user demand / analytics
3. Consider removing unsupported types from schema to avoid false advertising

[anand-testcompare]

Grooming update (2026-02-20): sequence is no longer the known broken case from #68.\n\nCurrent template map includes sequence as an explicit template (packages/backend/lib/templates/index.ts:13-18), and #113 closed #68.\n\nIssue is still valid for the remaining unimplemented diagram types, but description text should be treated as stale where it says sequence is broken.