swap x/y formatters for vline

[daschw]

vline uses y input and applies it to the x axis. This messes up the axis formatter for type recipes (cf #1762). This is a somewhat hacky workaround, but I think a special case for vline can be justified.

using Plots, Dates
plot(DateTime(2018):Day(1):DateTime(2019), cumsum(randn(366)))
vline!([DateTime(2018, 6)])

User-defined formatters still match the correct axis:

plot(rand(10))
vline!([4], yformatter = :scientific)

Closes #1762 and closes #1914.

[crbinz]

Any chance we can get a maintenance release for this? I have a long-standing issue in my own code that has been blocked by this and I'd love to close it out :)

[daschw]

Sure thing JuliaRegistries/General#3317