handle boxplot without y-variable in tinyplot.default

[zeileis]

Fixes #453

Like boxplot(x) it would be good if tinyplot(x, type = "boxplot") created a single boxplot without a grouping variable (and without corresponding labels on the x-axis).

I think that this is not easy to handle in type_boxplot() but requires a few simple workarounds in tinyplot.default() where the is.null(y) cases are handled. I've added:

    } else if (type == "boxplot") {
      y = x
      x = rep.int("", length(y))
      xlab = ""
      xaxt = "a"

With this both the default and the formula interface work:

tinyplot(Nile, type = "boxplot")
tinyplot(~ Nile, type = "boxplot")

Grouping and faceting still works:

tinyplot(~ bill_len | species, data = penguins, type = "boxplot")

tinyplot(~ bill_len, facet = ~ species, data = penguins, type = "boxplot")

[zeileis]

P.S.: It would be good to modularize this variable handling in the type_* functions somehow.

P.P.S.: While looking at data_boxplot() I noticed that ymin and ymax are set up but apparently never used. I think this could just be dropped?

https://github.com/grantmcdermott/tinyplot/blob/boxplot-nogroups/R/type_boxplot.R#L131-L132

[grantmcdermott]

Great, thanks for simple workaround @zeileis.

P.S.: It would be good to modularize this variable handling in the type_* functions somehow.

Agree. I worry it will require a larger refactoring b/c these exceptions come before the type-based datapoints construction. OTOH the current logic might just be inertia from the original design. In which case, moving to a modularized approach would kill two birds with one stone by nicely simplifying things.

P.P.S.: While looking at data_boxplot() I noticed that ymin and ymax are set up but apparently never used. I think this could just be dropped?

I might be mis-remembering (we should test), but I think we need those for the flip logic here. I have a vague memory of that anyway...

[grantmcdermott]

@zeileis Assuming you agree with my comments (which basically amount to "let's tackle these other issues separately"), do you mind adding a NEWS item? We can merge directly afterwards.

[zeileis]

OK, good, NEWS item added now.