Better support for trellis displays

There are a few different ways to do a trellis display currently, but these are all a bit esoteric

one_plot <- function(d) {
  plot_ly(d, x = ~wt, y = ~mpg)
}

mtcars %>%
  group_by(vs) %>%
  do(plot = one_plot(.)) %>%
  subplot(nrows = NROW(.))

plot_ly(mtcars, x = ~wt, y = ~mpg, color = ~factor(vs), yaxis = ~paste0("y", vs + 1)) %>%
  subplot(nrows = 2)

plot_ly(mtcars, x = ~wt, y = ~mpg, split = ~factor(vs), yaxis = ~paste0("y", vs + 1)) %>%
  subplot(nrows = 2)

#726 implements a trellis argument, which essentially maps that variable to sensible trace anchors, then uses subplot()'s machinery to populate axis objects. The problem is, we have guarantee a subplot is returned if we want a sensible result for:

plot_ly(mtcars, x = ~wt, y = ~mpg, trellis = ~vs)

Thus, we lose the ability to specify the number of rows in that subplot

It'd be better to have trellis() function:

trellis <- function(p, formula, ...) {
  # do some stuff here
  subplot(p, ...)
}

Ideally, the trellising would work a lot like ggplot2 facets (trellising variables should not appear in legends, and appear as annotations instead). This is what we do in ggplotly() via clever use of showlegend and legendgroup.

まず、#726 で提案されている trellis 引数と、既存の subplot() の仕組みを確認してください。完成した解決策では、subplot の行を制御でき、ggplot2 のような動作をサポートする trellis() 関数を提供する必要があります。これには、trellising 変数を凡例から除外し、注釈として表示することも含まれます。