spec: add streamgraph-basic specification

plots/streamgraph-basic/specification.md

@@ -0,0 +1,28 @@
+# streamgraph-basic: Basic Stream Graph
+
+## Description
+
+A streamgraph (also known as a stacked area chart with a centered baseline) displaying the composition of multiple categories over time with smooth, flowing curves. Unlike traditional stacked area charts, streamgraphs use a symmetric baseline centered around the x-axis, creating an organic, river-like appearance that emphasizes the overall shape and relative proportions of each category while minimizing the visual distortion of individual layers.
+
+## Applications
+
+- Music listening trends showing genre popularity over months
+- Topic popularity evolution in social media or news
+- Market share changes among competitors over time
+- Resource allocation across teams or projects over quarters
+
+## Data
+
+- `time` (datetime/numeric) - continuous time axis for the x-dimension
+- `category` (categorical) - distinct categories/series to stack
+- `value` (numeric) - magnitude for each category at each time point
+- Size: 10-100 time points with 3-8 categories
+- Example: monthly streaming hours by music genre over two years
+
+## Notes
+
+- Use smooth interpolation (spline/basis) for flowing curves
+- Center the baseline symmetrically around the x-axis
+- Use distinct, harmonious colors for each category
+- Include a legend to identify categories
+- Consider color palette that works well for adjacent areas

plots/streamgraph-basic/specification.yaml

@@ -0,0 +1,32 @@
+# Specification-level metadata for streamgraph-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: streamgraph-basic
+title: Basic Stream Graph
+
+# Specification tracking
+created: 2025-12-14T19:38:15Z
+issue: 856
+suggested: MarkusNeusinger
+history: []
+
+# Classification tags (applies to all library implementations)
+tags:
+  plot_type:
+    - streamgraph
+    - area
+    - stacked
+  domain:
+    - general
+    - media
+    - business
+  features:
+    - basic
+    - time-series
+    - composition
+  audience:
+    - intermediate
+  data_type:
+    - datetime
+    - categorical
+    - numeric