spec: add subplot-grid-custom specification

[github-actions]

New Specification: subplot-grid-custom

Related to #2856

---

specification.md

subplot-grid-custom: Custom Subplot Grid Layout

Description

A flexible subplot grid allowing multiple different plot types arranged in a custom grid layout with non-uniform cell sizes. Unlike regular subplot grids with uniform cells, this layout supports cells that span multiple rows and/or columns, enabling complex dashboard-style visualizations where some plots require more visual real estate than others.

Applications

• Executive dashboards with a large overview chart and smaller detail panels
• Scientific publications combining a main visualization with supporting annotations
• Financial reports with a primary price chart spanning multiple columns alongside volume and indicator panels
• Data exploration layouts with a large scatter matrix and smaller summary statistics plots

Data

• x (numeric or categorical) - Primary variable for each subplot
• y (numeric) - Secondary variable for each subplot
• z (numeric, optional) - Tertiary variable for 3D or color-mapped subplots
• Size: Varies per subplot, typically 20-500 points per cell
• Example: Dashboard with main time series (spanning 2 columns), volume bar chart, and returns histogram

Notes

• Support GridSpec-style layouts with colspan and rowspan parameters
• Allow cells to span multiple rows (e.g., a tall sidebar plot)
• Allow cells to span multiple columns (e.g., a wide header plot)
• Maintain consistent spacing and alignment despite varied cell sizes
• Clear visual hierarchy with larger plots for primary data and smaller for supporting views
• Example grid pattern: one 2x2 cell for main plot, four 1x1 cells for detail views

---

Next: Add approved label to the issue to merge this PR.

---

🤖 spec-create workflow