Colour legend is broken in scatterplot where only one value exists and a colorbar title is provided

Example:

library(plotly)
df <- data.frame(x = 1:5, y = 1:5, col = c(1,1,1,1,1))
plot_ly(df, x = ~x, y = ~y, color = ~col) %>%
    add_markers(marker = list(colorbar = list(title = "test")))

Result:

If any of the col values changes and it is no longer only 1's, then it works as expected. Also, if the title parameter is omitted, it also works as expected.

Here's the result when I omit the title:

This is using plotly 4.8.0 - latest on CRAN

运行提供的 plotly R 示例，使用恒定的 col 值和 colorbar 标题，然后将其与省略标题或 col 变化的版本进行比较。跟踪颜色图例和 colorbar 标题的行为，并在恒定值散点图能够正确渲染标题且不破坏图例时，将该 issue 视为完成。