R语言Plotly动画桑基图固定链路高度关闭自动重缩放方法问询

问题现象 #

使用R语言Plotly库制作动画桑基图时，每切换一帧Plotly都会自动重缩放链路高度，数值恒定的链路高度也会发生变化，所有链路总会填满100%的画布高度，比如示例中A2与B2之间的链路高度本应保持恒定。
目前临时通过创建哑节点占用空白空间的方式实现效果，但该方案不适用于复杂度超过Mike Bostock经典能源预测桑基图的场景。

原问题复现代码 #

library(plotly)

S.links <- data.frame(source = c(0,1,0,2,3,3, 0,1,0,2,3,3),
                    target = c(2,3,3,4,4,5, 2,3,3,4,4,5),
                    value =  c(8,4,2,8,4,2, 16,4,2,16,4,2),
                    year = c("2001","2001","2001","2001","2001","2001","2002","2002","2002","2002","2002","2002"))

plot_ly(
  type = "sankey",
  orientation = "h",
  
  node = list(
    label = c("A1", "A2", "B1", "B2", "C1", "C2"),
    color = c("blue", "blue", "blue", "blue", "blue", "blue"),
    pad = 15,
    thickness = 20,
    line = list(
      color = "black",
      width = 0.5
    )
  ),
  
  link = S.links,
  frame = ~S.links$year
)

可行解决方案 #

方案1：使用ids参数实现对象恒定性（推荐） #

实现逻辑

动画帧切换时Plotly默认按链路的位置顺序匹配对象，没有固定标识时会将每帧作为全新桑基图渲染，触发全局高度重缩放。给每条逻辑上唯一的链路分配跨帧通用的唯一id，Plotly就会将其识别为同一个对象，不会因为其他链路数值变化调整该链路的相对高度。

操作修改

1. 给链路数据集新增id列，相同逻辑的链路（比如A2→B2的链路）在所有年份共用同一个id
2. 在plot_ly的link参数中绑定ids = ~id
3. 新增arrangement = "fixed"参数，禁止节点位置自动调整

修改后代码

library(plotly)
library(dplyr)

S.links <- data.frame(source = c(0,1,0,2,3,3, 0,1,0,2,3,3),
                    target = c(2,3,3,4,4,5, 2,3,3,4,4,5),
                    value =  c(8,4,2,8,4,2, 16,4,2,16,4,2),
                    year = c("2001","2001","2001","2001","2001","2001","2002","2002","2002","2002","2002","2002")) %>%
  # 按source+target生成唯一链路id，跨帧通用
  mutate(id = paste0("link_", source, "_", target))

plot_ly(
  type = "sankey",
  orientation = "h",
  arrangement = "fixed", # 固定节点排布，禁止自动调整
  
  node = list(
    label = c("A1", "A2", "B1", "B2", "C1", "C2"),
    color = c("blue", "blue", "blue", "blue", "blue", "blue"),
    pad = 15,
    thickness = 20,
    line = list(
      color = "black",
      width = 0.5
    )
  ),
  
  link = list(
    source = ~source,
    target = ~target,
    value = ~value,
    ids = ~id # 绑定链路唯一id
  ),
  frame = ~year
)

方案2：全局数值归一化 #

实现逻辑

自动缩放的本质是每帧会将当前帧所有链路的总value映射为100%画布高度。提前统计所有帧的总value最大值，将每帧的所有链路value统一按该最大值做等比缩放，保证每帧的总value恒定，即可避免跨帧缩放。
该方案适合不需要严格保留原始数值绝对值，只需要展示相对比例的场景。

内容的提问来源于stack exchange，提问作者Julian