How to achieve color gradient across polyline in R

Question

I have a leaflet map with a polyline. I would like to give the polyline a color gradient from the beginning of line to the end of line. The color will not be dependent on any other variables. The line represents a trajectory, so I would like to make the direction of the trajectory visible with colors.

I have tried numerous things, but nothing is working.

The polyline is from a Line dataset (S4 object of class Line) that contains the coordinates under the name 'coords'. You can see the structure of it below:

enter image description here

This code works for the polyline with just one color:

map <- leaflet(data=longlatcoor)
map <- addTiles(map)
map <- addCircles(map, data=longlatcoor, radius=2)

sl1 = Line(longlatcoor)
map <- addPolylines(map,data=sl1, color = "green")
map 

The longlatcoor is a SpatialPoints dataset. I cannot get a gradient color on the polyline, no matter what I do.

I am new to R and I struggle to understand the error messages and what else I can do.

I tried this (code is copied from another site, except the name of my sl1 dataset):

gradientFunction <- colorRampPalette(c("white", "blue"))
colorGradient <- gradientFunction(dim(df)[1])

df1 <- sl1 %>%
  mutate(nextLat = lead(lat),
     nextLng = lead(lng),
     color = colorGradient
     )

gradient_map <- leaflet() %>% 
  addTiles()

for (i in 1:nrow(df1)) {
  gradient_map <- addPolylines(map = gradient_map,
                           data = df1, 
                           lng = as.numeric(df1[i, c('lng', 'nextLng')]), 
                           lat = as.numeric(df1[i, c('lat', 'nextLat')]), 
                           color = as.character(df1[i, c('color')])
  )
}

gradient_map

But I get this error when I try to create df1: no applicable method for 'mutate' applied to an object of class "Line"

I have also tried this:

pal <- colorNumeric(
  palette = "Blues",
  domain = sl1@coords)

leaflet() %>% 
  addTiles() %>% 
  addPolylines(data = df, lng = ~lng, lat = ~lat, 
               color = ~pal(coords)) 

but get this error:

no applicable method for 'metaData' applied to an object of class "function"