Revert call to cat_1_arg() in plot.

[joa-quim]

Update some docs.

[codecov-io]

Codecov Report

Merging #174 into master will decrease coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   81.35%   81.18%   -0.18%     
==========================================
  Files          78       78              
  Lines        3401     3402       +1     
==========================================
- Hits         2767     2762       -5     
- Misses        634      640       +6

Impacted Files	Coverage Δ
src/psxy.jl	83.47% <100%> (-2.55%)	⬇️
src/plot.jl	80.91% <100%> (ø)	⬆️
src/common_options.jl	84.55% <100%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4350d8f...9e62da1. Read the comment docs.