layer_lines fails where layer_points succeeds in adding overlay to vis

[dholstius]

By "overlay" I mean a layer based on an auxiliary dataset. Simple example using mtcars:

library(ggvis)

my_vis <-
  ggvis(mtcars, x = ~mpg, y = ~wt) %>%
  mutate(cyl = factor(cyl)) %>%
  group_by(cyl) %>%
  layer_lines(stroke = ~cyl)

# Some (fictional, toy) auxiliary data
foo_data <- data.frame(
  mpg = seq(12, 30, len = 7),
  wt = seq(5, 2, len = 7))

# To superimpose auxiliary data on a vis
add_overlay <- function (.vis, overlay_data, overlay_layer = layer_points, color = "black") {
  overlay_layer(.vis, data = overlay_data, stroke := color, fill := color)
}

my_vis %>% add_overlay(foo_data, layer_points, "black")  # `layer_points` succeeds
my_vis %>% add_overlay(foo_data, layer_lines, "black")   # `layer_lines` FAILS