legend positioning

a lot of legends are coming out with weird positioning (https://github.com/ropensci/plotly/issues/194#issuecomment-87780687)

a couple things to work out:

• move the horizontal position of the legend to the right more

  layout$legend$x <- 1.0 # move it to the right a bit
  layout$legend$xanchor <- "left" 
  layout$annotation[[1]]$x <- 1.01
  layout$annotation[[1]]$xanchor <- "left"
  

• the vertical position of the legend should stay in the middle, with yanchor <- "middle"

  layout$legend$y <- 0.5 # page middle
  layout$legend$yanchor <- "middle" 
  

  but we should compute the vertical position of the legend title annotation to remain above the legend, no matter how many entries. For now, we could hardcode a mapping in that would work for default font-sizes:

  • 8 legend entries: annotation$y <- 0.65 (from manual inspection)
  • 1 legend entry: annotation$y <- 0.52 (manual inspection)
  • -> annotation$y <- 0.14125 * number_of_legend_items - 0.48 (computed from those two points above)

  @alexcjohnson, how do you think legend titles will work in the future? a separate attribute inside legend?

• manual legend positions from ggplot: https://github.com/ropensci/plotly/blob/add-r-cookbook-tests/tests/cookbook-test-suite/legends.R#L109-L122

cc @tdhock , @cpsievert