dyRibbon中调色板颜色的顺序是什么？

Question

Consider these questions around a reprex using dyRibbon function of dygraphs package in R.

Q1. Why do we need 4 colors for a 3 ribbon display? Why is the first one always ignored?

Q2. How is the sequence of the ribbon color palette assigned to each numerical value? It is surely not per the numerical order as you will see in the reprex below

library(data.table)
library(dygraphs)
library(RColorBrewer)
library(dplyr)

set.seed(12)
# create data
dt1 <- data.table(t = seq(Sys.time(),by  = "1 sec", length.out = 10),
                  y = rnorm(10,20,5))
# add a column with 3 possible values to be used for dyRibbon
dt1[,flag1:=factor(dplyr::case_when(y>22 ~ "high",y>18 ~ "med", T ~ "low"))]
dt1[,rflagn:=as.numeric(flag1)]

# generate the dygraph with a 3 color ribbon
dyg1 <- 
  dt1 %>% 
  select(t,y) %>% 
  dygraph(main = "Reprex to show color sequence") %>% 
  dyOptions(drawPoints = T,pointSize = 5,drawGapEdgePoints = T,
            strokeWidth = 4,strokeBorderWidth = 1) %>% 
  dyRibbon(dt1$rflagn/4,palette = brewer.pal(4,"Set1"))

# checkout the reprex graph paying attention to the sequence of colors
dyg1

# check the sequence of colors in the palette
display.brewer.pal(4,"Set1")

# check the actual values in first 10 rows
dt1
#>                       t        y flag1 rflagn
#>  1: 2022-05-18 21:47:10 12.59716   low      2
#>  2: 2022-05-18 21:47:11 27.88585  high      1
#>  3: 2022-05-18 21:47:12 15.21628   low      2
#>  4: 2022-05-18 21:47:13 15.39997   low      2
#>  5: 2022-05-18 21:47:14 10.01179   low      2
#>  6: 2022-05-18 21:47:15 18.63852   med      3
#>  7: 2022-05-18 21:47:16 18.42326   med      3
#>  8: 2022-05-18 21:47:17 16.85872   low      2
#>  9: 2022-05-18 21:47:18 19.46768   med      3
#> 10: 2022-05-18 21:47:19 22.14007  high      1

Answer 1

I got the answer in rstudio community. The solution is the sequence has to be explicitly defined.

data_arg <- dt1 %>% 
    mutate(flagn = case_when(
        flag1 == "low" ~ 0,
        flag1 == "med" ~ 0.5,
        flag1 == "high" ~ 1)) %>% 
    pull(flagn)

Once this is defined we just pass it to dyRibbon .

...
dyRibbon(data = data_arg, palette = brewer.pal(3,"Set1"))

So I was relying on the default sequence of the factor which could be arbitrary. It is better to define the data argument explicitly.

Thanks to https://community.rstudio.com/t/how-does-the-sequence-of-palette-color-in-dyribbon-get-decided/137559/2?u=sanjmeh