fix numeric xlevels specification in type_barplot

[zeileis]

Fixes #430

• Numeric xlevels specifications in type_barplot() never worked correctly because the x rather than datapoints$x was used in one place. Fixed now.
• Improved the documentation of xlevels and added two simple examples.
• Added a warning in cases where xlevels contains entries that do not correspond to any level of x.

[vincentarelbundock]

Thanks for the fix! Maybe insert a test like this:

f = function() {
  tinyplot(
    ~cyl,
    data = mtcars,
    type = "barplot",
    beside = TRUE,
    xlevels = levels(factor(mycars$cyl))[3:1]
  )
}
expect_snapshot_plot(f, label = "barplot_xlevels_issue430")

[zeileis]

Good suggestion, thanks, added now (but using an example that failed before the fix).

[zeileis]

Similar improvements have now been done for type_spineplot() as well where xlevels was also added (previously we only had ylevels).

[vincentarelbundock]

I looked at the code and it seems great to me. I think we can merge.

[zeileis]

Great, thanks for the feedback, this is very helpful. Should we wait for feedback from the original poster?

[vincentarelbundock]

I'm usually too impatient for that, but maybe it's a good idea :)

[zeileis]

OK, I will let her know when the dev version is on R-universe.

[grantmcdermott]

Thanks all, LGTM.