Draw line shapes outlining brush

line-on-brush.R

library(plotly)
library(crosstalk)
library(htmlwidgets)

mtcars %>% 
  SharedData$new() %>% 
  plot_ly(x = ~wt, y = ~mpg) %>% 
  highlight("plotly_selected", dynamic = TRUE) %>%
  onRender(
"function(el, x) {
  var gd = document.getElementById(el.id);
  gd.on('plotly_selected', function(ed) {
    var xmax = ed.range.x[1];
    var ymin = ed.range.y[0];
    var vline = {type: 'line', y0: 0, y1: 1, yref: 'paper', x0: xmax, x1: xmax, line: {dash: 'dot'}};
    var hline = {type: 'line', x0: 0, x1: 1, xref: 'paper', y0: ymin, y1: ymin, line: {dash: 'dot'}};
    Plotly.relayout(gd, {shapes: [vline, hline]})
  })
}"
)