Log Message:
tex-pgfplots{,-doc}: update to 1.18.1

1.18.1
- fixed a compatibility issue with PGF that slipped in right after
running the release tests.

1.18
- new feature: 'contour lua', implemented and contributed by
Francesco Poli
- new feature: ignore empty coordinates when computing regression
lines
- new feature: improve log-regression fitting of exponential
functions (using variance format=linear)
- fixed bug: image plots in log mode failed to work. Also fixed
issues with mesh/ordering=y varies
- fixed bug: handle NaN in stacked plots