don't overwrite custom formatter for with vline!

[daschw]

fix #4538

plot(rand(4) .* 10^6, rand(4) .* 10^6, xformatter=:plain, yformatter=:plain)
vline!([10^6])

[codecov]

Codecov Report

Base: 90.65% // Head: 90.78% // Increases project coverage by +0.13% 🎉

Coverage data is based on head (6c8902b) compared to base (3fe21ae).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4619      +/-   ##
==========================================
+ Coverage   90.65%   90.78%   +0.13%     
==========================================
  Files          41       41              
  Lines        8772     8773       +1     
==========================================
+ Hits         7952     7965      +13     
+ Misses        820      808      -12     

Impacted Files	Coverage Δ
src/args.jl	84.98% <100.00%> (ø)
src/backends/gr.jl	91.66% <0.00%> (+<0.01%)	⬆️
RecipesPipeline/src/utils.jl	97.26% <0.00%> (+1.36%)	⬆️
RecipesPipeline/src/series.jl	93.58% <0.00%> (+2.56%)	⬆️
src/examples.jl	99.20% <0.00%> (+3.17%)	⬆️
RecipesPipeline/src/RecipesPipeline.jl	100.00% <0.00%> (+11.11%)	⬆️

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