PPTX: preserve native chart layout and value formatting

[developer0hye]

Problem

The PPTX chart fallback does not preserve the native chart composition. The title is detached and wrapped vertically, the chart is stretched into a thin frame, value labels expose raw floating-point data, and legend/axis styling differs substantially.

Evidence

Microsoft PowerPoint GT	office2pdf Before

Expected

Preserve chart title, plot-area proportions, category order, axes/gridlines, legend, series styling, and displayed number formatting.

Related: #213
Discovered by: #216

[developer0hye]

Fixed by PR #323 (merged).

Bar and column charts now render as axis-scaled plots: a nice-rounded value axis with gridlines and tick labels, category labels, axis-scaled bars (drawn bottom-up for bar charts), a right-side legend, the chart/series title on top, and Office accent series colors. Value labels are formatted to drop float noise (8.2000001 → 8.2), and bars carry no in-plot labels — matching PowerPoint. Verified on the bar-chart fixture against PowerPoint GT (see the PR's GT/before/after).

Line/area/scatter charts keep the existing fallback; the XLSX line-chart case is tracked in #239.