[pierrevbc]

Problem

Since #355, an unchanged chart keeps the embedded workbook it was read with (and the other parts
its relationships reach, in Chart.kept) when the presentation is saved as PPTX. What is still
lost:

• Any edit drops the workbook. Every chart command (chart.type, chart.data,
  chart.options) clears Chart.raw, so after a title change, a new legend or one changed value
  the chart is no longer "unchanged" and is saved without c:externalData or a workbook. Edit Data
  in other presentation apps has nothing to open, and the workbook's other sheets, formulas and
  formatting are gone.
• Charts made in DeckCraft have no workbook at all.

Reproduce on main with a public file, not for committing:

1. features/steps/test_files/cht-charts.pptx from scanny/python-pptx (commit 278b47b), or
   test-data/slideshow/aascu.org_hbcu_leadershipsummit_cooper_.pptx from apache/poi (slides 6
   and 7, one chart each).
2. deckcraft-cli run --in cht-charts.pptx --cmd 'chart.options={"title":"New"}' --export out.pptx
   (any chart command), or insert a chart and export.
3. unzip -l out.pptx | grep embeddings prints nothing for that chart; its chartN.xml has no
   c:externalData.

Design

The workbook stays where #355 put it: in Chart.kept, the package parts an imported chart points
at. No new media item, no new field for it in the model. ChartKept::workbook() finds the kept
workbook part (the internal package or oleObject target of the chart's relationships).

On export, Exp::chart_rel:

1. Unchanged chart (its kept part still describes the model): Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355's path, untouched. The
   original part goes back with every kept part under fresh keptN names.
2. Edited chart whose data still fits its original ranges (Chart.source, commit 2): the
   chart part is written from the model with its original formulas, and the kept workbook is
   written next to it: byte for byte when only the presentation changed (title, type, legend…),
   with only the changed cells rewritten when the data changed.
3. Anything else (a chart made in DeckCraft, series or points added or removed…): a workbook is
   generated from the chart data, laid out where the generated chart part's formulas point, with
   its c:externalData.

An Office 2016 chart's plain-chart fallback gets no workbook, as with #355: it belongs to the
chartEx part.

What changes

Two commits, then a separate one adding my entry to contributors/people.toml
([people.pierrevbc], display name only).

1. pptx: write a workbook for edited and new charts

• Model. ChartKept::workbook().
• Export. Cases 1 and 3 above: an edited or new chart gets a minimal generated workbook (one
  sheet, Sheet1, inline strings; categories in column A, one column per series, series names on
  row 1) under ppt/embeddings/workbook{n}.xlsx, a package relationship and
  c:externalData.
• Layout fix. Scatter series with different X values, and bubble charts with several series,
  get columns of their own. Before, every scatter series pointed at column A for its X values and
  bubble sizes used col(i + 2), which overlapped the next series' values.
• Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355's tests in chart_embed.rs: the fixtures strip the workbook the writer now generates
  for a new chart (to stand for a file from an app that writes none);
  an_edited_chart_does_not_write_a_stale_workbook now checks that the stale bytes are not
  written and that the chart gets one workbook of its own data.

2. pptx: update chart workbooks in place instead of rebuilding them

• Model. Chart.source: Option<ChartSource>: for each series of an imported chart, the
  formulas of its name, categories (or X values), values and bubble sizes (c:f), and the data the
  chart part cached for them. Commands leave it alone, unlike raw. Skipped when absent, so
  existing .deckcraft files serialize as before.
• Export, data unchanged. When the data still fits the original ranges (same series and point
  counts, same category or XY family, formulas present), the chart part is written with its
  original formulas and the kept workbook is written back byte for byte.
• Export, data changed. Only the cells whose data differs from what was read are rewritten in
  the kept workbook (crates/pptx/src/write/xlsx.rs, quick-xml and zip, no new dependency, no
  file or network I/O). Every other zip entry is copied with its compressed bytes. In the
  workbook:
  • a cell keeps its style; text is written as an inline string;
  • a new header in a table renames the table column, which Excel requires;
  • a value typed over a formula replaces it, and calcChain.xml (with its relationship and
    content type) is dropped so Excel does not repair the file; Excel rebuilds it;
  • fullCalcOnLoad="1" is set on calcPr when the workbook holds formulas, so formulas
    elsewhere follow the new values.
• Fallback to a generated workbook (case 3, with a warning in the log) when: series or points
  were added or removed; a category chart became a scatter or bubble chart (or back); the chart
  part held literal data; a formula is not one row or column of one sheet (unions, names, other
  workbooks); a target cell holds a shared or array formula; a new table column name is empty,
  clashes with another column, or appears in a structured reference; a linked workbook's data
  changed; the workbook is an older OLE embedding.

How it was tested

cargo xtask ci passes on this branch, on main at 9859da3 (all 7 steps: fmt, pdf-deps, clippy
-D warnings, 1,687 tests passed and 6 ignored, assets, layers, wasm32 check). On Windows the
deckcraft-ui-egui lib tests sometimes crash with an access violation when run in parallel, on
main as well; the run above used RUST_TEST_THREADS=1.

New and changed tests, all on decks and workbooks built in code:

• write::chart unit tests: for 11 chart variants (column, stacked bar, line, pie, radar, combo,
  scatter with shared and with separate X values, single-series bubble, multi-series bubble with
  shared and with separate X values), every cached value in the chart part equals the workbook
  cell its formula points to; an unchanged chart keeps its part; edited data leaves that path.
• write::xlsx unit tests: formula parsing (quoted sheet names, one row or one column only),
  cells replaced and inserted in order (including new rows and an empty sheetData), shared
  formulas and implicit positions refused, calcPr set or inserted at its schema position, table
  columns renamed.
• tests/chart_workbook.rs, on a workbook built by the test (a Chart data sheet with a table, a
  total in F2, a formula in D5, a Notes sheet with a formula into the first one, shared strings,
  a calculation chain): title, legend and type changed keep the workbook byte for byte; one value
  changed rewrites only that cell and sets fullCalcOnLoad; a renamed series renames the table
  column; a value over a formula drops the formula and the calculation chain; added series or
  points and a clashing series name rebuild the workbook; a scatter chart updates in place; a chart
  made in DeckCraft gets a generated workbook, updated in place after a re-import.
• tests/roundtrip.rs and tests/chart_embed.rs (Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355): workbook bytes across round trips,
  unchanged charts still keep their parts under keptN names, the chartEx fallback has none.

Checked by hand earlier on the same logic (0.5.0, public decks above, not committed): changing the
title and legend keeps every workbook byte for byte; changing one value changes one cell of one
sheet, every other workbook entry (thumbnail included) staying identical; openpyxl and LibreOffice
read the updated workbooks.

Not verified: PowerPoint's Edit Data and Excel's recalculation on the updated workbooks. That needs
a black-box check on a machine with PowerPoint (AGENTS.md §2).

Known limits

• The workbook lives in Chart.kept, which a .deckcraft save doesn't store (Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355): a deck saved
  as .deckcraft and reopened has no kept workbook, so an export writes a generated one.
• The fallback cases above still lose the original workbook's extra sheets, formulas and
  formatting, and an edited chart drops the other kept parts (styles, drawings), as with Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355.
• An edited chart part is rebuilt from the model: options the model doesn't hold are lost, as
  before. Only the workbook is kept.
• A changed category is written as text, even over a number or date cell.
• Structured references are found by their text ([Name], [@Name]); escaped names in formulas
  may be missed. dimension and row spans hints are not updated.

Related PRs

• Charts: keep the embedded workbook when a chart is saved unchanged (#335) #355 (merged): an unchanged chart keeps its workbook and kept parts. This PR builds on its
  ChartKept and leaves its unchanged-chart path as it is.
• Charts: every LibreOffice chart type, a data editor, and formatting for each #255 (every chart type, data and formatting) stacks on this PR.
• git merge-tree against main at 9859da3 is clean.

[Harrison-Booth]

An embedded workbook would let users edit chart data in PowerPoint, closing the high-priority chart interoperability gap. If you would rather not, I can open a PR that credits you, keeps your commits and shows a git range-diff against this one, so you'll still get credit and have visible work.