For a categorical colour aesthetic, there appears to be no documented way to use selectedpoints to select individual points.

The following example highlights the first point from each colour group (the first "a" point and the first "b" point):

library(plotly)

d <- data.frame(x = 1:4, c = c("a", "a", "b", "b"))

plot_ly(d) |>
  add_trace(x=~x, y = ~x, color = ~c, type = "scatter", selectedpoints = list(0))

From the plot_ly() documentation, it appears that this code implicitly creates multiple traces, one for each colour, so this behaviour makes sense on this level (even if it's surprising). However, there appears to be no documented way to access individual traces. I've tried a number of permutations

In my opinion, if the code is going to transparently create multiple traces like that, it should also transparently map selectedpoints provided by the user (ideally as 1-based indexes or logical vectors, for consistency with R) to their positions within each trace.

从 plot_ly() 文档和分类颜色示例开始，追踪 colour 如何创建单独的 trace，以及 selectedpoints 如何传递。完成的标准是提供一种有文档说明且一致的方式，用于选择每个生成的 trace 中的点，并覆盖所示示例和提议的 R-style 索引行为。