ggvis add_legend用于'strokeDash'或'strokeWidth'

Question

我想将两个变量的数据分组，并通过'stroke'和'strokeDash'在ggvis图中区分它们。如果我想在ggvis图中添加图例，我可以为'stroke'属性添加：

library(ggvis)

mtcars %>% 
  ggvis(~wt, ~mpg, stroke = ~factor(cyl), strokeDash = ~factor(vs)) %>%
  layer_lines() %>%
  add_legend('stroke', title = 'Number of cylinders')

但是，如果尝试为'strokeDash'添加图例，则不会渲染图：

mtcars %>% 
  ggvis(~wt, ~mpg, stroke = ~factor(cyl), strokeDash = ~factor(vs)) %>%
  layer_lines() %>%
  add_legend('stroke', title = 'Number of cylinders') %>%
  add_legend('strokeDash', title = 'V/S')

同样适用，如果我使用'strokeWidth'代替'strokeDash'：

# works
mtcars %>% 
  ggvis(~wt, ~mpg, stroke = ~factor(cyl), strokeWidth = ~factor(vs)) %>%
  layer_lines() %>%
  scale_nominal('strokeWidth', range = c(1,5)) %>% 
  add_legend('stroke', title = 'Number of cylinders')

# does not work
mtcars %>% 
  ggvis(~wt, ~mpg, stroke = ~factor(cyl), strokeWidth = ~factor(vs)) %>%
  layer_lines() %>%
  scale_nominal('strokeWidth', range = c(1,5)) %>% 
  add_legend('stroke', title = 'Number of cylinders')%>%
  add_legend('strokeWidth', title = 'V/S')

有谁知道，为什么“中风”有效，而有些则没有？任何解决方案？

谢谢， 费边