Read and write embedded charts

Cluster: charts

Scenario

Users want to read and write embedded charts (bar, column, line, pie, doughnut, scatter, area, radar, …) inside a workbook. A chart lives as a DrawingML chart part anchored to a worksheet drawing, referencing worksheet cell ranges as its data series and category axis. Consumers want to open an application-produced workbook and enumerate or inspect its charts, and to declare new charts that a spreadsheet application opens without repair and renders from live cell data. Preserving an unmodeled chart part verbatim across a round-trip is already captured by chart-parts-survive-template-roundtrip; this note is the larger ask, a first-class, typed chart model.

Spec note, not a corpus case: this is a substantial new feature with no failing behavior to assert yet. The durable value is the read/write chart model and its open design questions.

Desired behavior

• Read path: parse the chart parts (DrawingML chart XML under the drawing relationships) and surface a typed chart model per worksheet: chart kind, title, plot area, one or more data series with their value, category and name range references, axis configuration, and the drawing anchor (cell-anchored position and size) tying the chart to the sheet.
• Write path: declare a chart on a worksheet by kind, series (each pointing at a cell range for values, optionally categories and a series-name cell), title, and anchor. On write, emit a valid chart part, the drawing part, and all relationships so the file opens without repair and renders from live worksheet data.
• Round-trip: a chart present in a loaded workbook survives a read-then-write unchanged, both in the passthrough case and, once modeled, the typed case, with series range references intact.

Open questions

• How much of the chart type space is first-class rather than passthrough-only initially? Bar, line, pie and scatter cover most demand.
• Series data: only cell-range references (live data), or also inline or cached literal values?
• Is the chart model immutable-by-default with explicit mutation entry points, matching the fork's stance, and how does it compose with the drawing and anchor model shared with images?
• Combo charts, secondary axes, and stock, radar and surface types: in scope or deferred?

Related: chart-parts-survive-template-roundtrip, pivot-table-parts-survive-roundtrip, image-anchor-emu-from-real-column-geometry.