Shiny App中动态设置ggvis的viridis调色板颜色数量

可以用命名向量建立选项与颜色数量的映射，这种方式简洁易维护，后续新增选项时只需扩展向量即可：

vis <- reactive({
  # 建立选项到颜色数量的映射关系
  color_n <- c(
    "Region" = 6,
    "Language" = 30,
    "Institution" = 100
  )
  
  # 根据当前用户选择获取对应颜色数量
  selected_n <- color_n[input$z]
  
  dataset %>%
    ggvis(x = ~Date, y = ~mentions, fill = ~get(input$z)) %>%
    layer_points(size := 50, size.hover := 200, 
                 fillOpacity := input$alpha, fillOpacity.hover := 1, key := ~Filename) %>%
    scale_ordinal("fill", range = viridis(n = selected_n)) %>%
    add_tooltip(speech_tooltip, "hover") %>%
    add_axis("x", title = "Date") %>%
    add_axis("y", title = paste0("Mentions of `", req(input$keyword), "'"))        
})

如果需要更灵活的条件判断逻辑，也可以用dplyr::case_when实现：

vis <- reactive({
  selected_n <- case_when(
    input$z == "Region" ~ 6,
    input$z == "Language" ~ 30,
    input$z == "Institution" ~ 100,
    # 可选：设置默认值，应对未匹配的异常情况
    TRUE ~ 10 
  )
  
  dataset %>%
    ggvis(x = ~Date, y = ~mentions, fill = ~get(input$z)) %>%
    layer_points(size := 50, size.hover := 200, 
                 fillOpacity := input$alpha, fillOpacity.hover := 1, key := ~Filename) %>%
    scale_ordinal("fill", range = viridis(n = selected_n)) %>%
    add_tooltip(speech_tooltip, "hover") %>%
    add_axis("x", title = "Date") %>%
    add_axis("y", title = paste0("Mentions of `", req(input$keyword), "'"))        
})

补充说明 #

• 命名向量方式适合固定选项的映射场景，修改和扩展成本极低；
• case_when方式支持更复杂的多条件判断，比如后续如果需要根据数据集实际类别数动态调整，只需替换固定数值为n_distinct(dataset[[input$z]])即可；
• 两种方案都能实现根据用户选择动态匹配viridis调色板的颜色数量，完全符合你的需求。

内容的提问来源于stack exchange，提问作者jeanmichop