Radar property for a Billboard.js chart
Arguments
- bb
- A - billboard- htmlwidgetobject.
- ...
- See https://naver.github.io/billboard.js/release/latest/doc/Options.html#.radar 
Examples
library("billboarder")
data("avengers")
# number of levels
billboarder() %>%
  bb_radarchart(
    data = avengers,
    mapping = bbaes(x = axis, y = value, group = group)
  ) %>% 
  bb_radar(level = list(depth = 4))
# hide levels
billboarder() %>%
  bb_radarchart(
    data = avengers,
    mapping = bbaes(x = axis, y = value, group = group)
  ) %>% 
  bb_radar(level = list(show = FALSE))
# max value on axis
billboarder() %>%
  bb_radarchart(
    data = avengers,
    mapping = bbaes(x = axis, y = value, group = group)
  ) %>% 
  bb_radar(axis = list(max = 10))