fix(pptx): preserve data points for scatter and bubble charts

[MeiSiristhebest]

Summary

markitdown silently dropped all data when converting PPTX scatter and bubble charts to Markdown.

Root cause

_convert_chart_to_markdown builds the Markdown table by iterating over chart.plots[0].categories. Scatter/bubble charts have no category axis, so categories is empty and the row loop ran zero times — only the header was emitted and every data point was lost.

Fix

Detect XY/bubble chart types via XL_CHART_TYPE and render rows point-by-point instead of row-by-category:

• The first column is the point's X value, read from the internal c:xVal element (python-pptx only exposes the Y values publicly via series.values).
• For bubble charts, an additional "(size)" column per series shows the bubble size.

Category charts (column, bar, line, pie, doughnut, radar, area, …) keep their original behavior unchanged. The O(n^2)→O(n) materialization of series.values is preserved.

Testing

Added packages/markitdown/tests/test_pptx_chart.py, which builds in-memory scatter and bubble charts and asserts the data points (and bubble sizes) appear in the converted Markdown. Both new tests pass and the existing test_pptx_svg.py still passes. Verified locally with pytest and black.

Fixes the data-loss behavior when converting real-world scatter/bubble charts.

[MeiSiristhebest]

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