Enhancement-557: pyplot_status on every pyplot, a colliding -expr name refused, a bin count below 2 said

pyplot_status on every pyplot, a colliding -expr name refused, a bin count below 2 said

Scope: F6, F7 and F8 of the
bug hunt of 2026-09-05:
src/frontend/com_pyplot.c, src/frontend/plotting/pyplot.c,
src/frontend/com_sweep.c. ngspice only; the compiler is unchanged.

Suites: pyplot_examples 49 → 53,
mcrecord_examples 13 → 14, both solvers;
the twelve neighbouring pyplot and Monte Carlo suites pass; full sweep 459 of
459 (paramgiven_examples now takes its
pre-E-555 compiler from the repository's history, since the CI republishes
bin/ after every push). The pyplot reference
(§13.1, the settings
table, the decimate row), the
statistics guide §6.1.

What was wrong

• F6. E-547 promised that every pyplot publishes pyplot_status, and
  five paths did not — a refusal before the backend ran (a usage error,
  plotit's own X values must be > 0 for log scale), a table or script that
  could not be opened (a read-only file, a missing directory), and a
  successful -export — so the deck's if $pyplot_status ne 0 died with
  no such variable on exactly the failures it was written to catch.
• F7. montecarlo refused two -expr of one name and let one take the
  record plot's own — sample, its scale, or a result such as
  montecarlo_n — which then shadowed the record: print sample read 1..N
  and the values were unreachable.
• F8. set pyplot_decimate=1, 0 or -5 silently turned decimation off
  where abc was told it was neither off, auto nor a bin count.

What changed

• com_pyplot assumes a failure at entry (pyplot_status = 1) and every path
  that succeeds says so: the launch path with Python's exit status, the export
  path with 0. A refusal, an unopenable table or script, a usage error leave 1.
• A -expr name the record plot owns is refused with the reason: -expr name
  'sample' is the record plot's own vector (its scale sample, or a result
  such as montecarlo_n); choose another name.
• A pyplot_decimate count below 2 gets the same warning as a non-number and
  decimation stays automatic.

Verification

check	result
pyplot -export exp2 v(in)	pyplot_status = 0, the table written
pyplot bad v(in) xlog xlimit 0 1m	X values must be > 0 for log scale, status 1
pyplot nodir/x v(in), an export onto a read-only file	status 1
set pyplot_decimate=1	is not a bin count (2 or more), off or auto; decimating automatically, the plot rendered
-expr sample=…, -expr montecarlo_n=…	refused, nothing recorded, the script goes on
pyplot_examples, mcrecord_examples; full sweep	53 / 53, 14 / 14; 459 of 459

Write-up: enhancements_doc/Enhancement-557.md.