Adding labels to polylines increases line opacity

[rbechtel-MITRE]

Adding labels or popups to polylines increase the line's opacity. The increase in opacity seems to be related to the number of points used to plot the polyline as if each time a label is added the opacity bumps up. It seems you can account for this by decreasing the opacity proportional to the number of points that makeup the polyline.

library(leaflet)

data <- tibble(x = c(10,20,30,40), y = c(10,20,30,40), label = rep("label",4))

#has default opacity
m <- leaflet() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolylines(data = data, lng = ~x, lat = ~y)

#has opacity closer to 1
m <- leaflet() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolylines(data = data, lng = ~x, lat = ~y, label = ~label)

#decreases opacity 
m <- leaflet() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolylines(data = data, lng = ~x, lat = ~y,# label = ~label,
               opacity = 0.5 / nrow(data), fill = 0.2 / nrow(data)) #default values divided by rows in data

[jack-davison]

I think this is because leaflet recycles the label four times - this works fine:

leaflet() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolylines(
    data = data,
    lng = ~ x,
    lat = ~ y,
    label = ~ label[1]
  )

Or see this version, where you can toggle between the different labels.

data <- tibble(x = c(10,20,30,40), y = c(10,20,30,40), label = paste0("label", 1:4))

#has default opacity
leaflet() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolylines(data = data, lng = ~x, lat = ~y, label = ~label, group = ~label) %>%
  addLayersControl(baseGroups = data$label)

If you want different labels per segment, you could do something like:

data <-
  tibble(
    x = c(10, 20, 30, 40),
    y = c(10, 20, 30, 40),
    label = paste0("label", 1:4),
    color = c("red", "blue", "green", "orange")
  )

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

for (i in seq_along(data$label)) {
  if (i > 1) {
    m <- 
      addPolylines(
        map = m,
        lng = data$x[(i-1):i],
        lat = data$y[(i-1):i],
        label = data$label[i],
        color = data$color[i]
      )
  }
}

print(m)

[rbechtel-MITRE]

Yep, adding [1] resolves the issue for me.