Add pievisible pipe #473

What happened?

Pie charts with many segments need a computed Other slice that updates when users toggle visibility in Options. selectattr:'visible' only filters rows — it can’t aggregate hidden segments into a single Other slice.

Use case: a pie chart with primary segments shown individually and secondary segments collapsed into Other by default; toggling a segment in Options promotes it and shrinks Other.

Expected: chart_segments | pievisible returns visible slices plus one Other slice summing hidden values (30 above). When MX is toggled visible, MX appears as its own slice and Other drops to 10. When everything is visible, Other is omitted.

Actual: no built-in pipe for this — authors must precompute slices in Python or hand-roll aggregation in expressions.

Proposed approach (comment on the issue if filing before the PR):

Add a pievisible pipe in renderer/src/expression.ts that:

takes segments with label, value, visible, and group (primary / secondary)
returns {label, value} slices for visible rows
aggregates hidden rows into a single Other slice

Example Code

Version Information

0.0.1.dev584+5cb520a