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")