Is there a way to conditionally vary opacity of polygons using Leaflet? - r

Question

I am using the R leaflet package to plot data for California counties. I would like the polygons of each county to vary in opacity based on the number of parcels in that county. Counties with more parcels should be more opaque, and counties with fewer parcels should be more transparent. Is this possible?
I have tried changing the fillOpacity option similar to how fillColor varies with number of parcels:
fillOpacity = ~num.parcels
Sample of my data:
packages <- c('dplyr','leaflet','sf','USAboundaries')
lapply(packages, library, character.only = TRUE)
ca_counties <- USAboundaries::us_counties(states = 'CA')
parcels <- structure(list(county = c("Yuba", "Sacramento", "Inyo", "Los Angeles", "Sierra"),
num.parcels = c(27797L, 452890L, 6432L, 15830L, 54291L)), row.names = c(NA, -5L), class = "data.frame")
parcels <- st_as_sf(left_join(parcels, ca_counties[,c('name')], by = c("county" = "name")))
Leaflet map:
labels <- sprintf(
"<strong>%s County</strong><br/>
Parcels: %g<br/>",
parcels$county, parcels$num.parcels
) %>% lapply(htmltools::HTML)
leaflet(parcels) %>%
setView(-119, 37.9, 6) %>%
addTiles() %>%
addPolygons(
fillColor = ~pal(num.parcels),
weight = 2,
opacity = 1,
color = 'black',
dashArray = '2',
fillOpacity = 0.7,
highlightOptions = highlightOptions(color = "red", weight = 3,
bringToFront = TRUE),
label = labels,
labelOptions = labelOptions(
style = list("font-weight" = "normal", padding = "4px 8px"),
textsize = "15px",
direction = 'auto')) %>%
addLegend(pal = pal, values = ~num.parcels, opacity = 0.7, title = "Number of Parcels",
position = "bottomleft")

Answer 1

try setting fillOpacity = ~num.parcels / max(num.parcels),
I also included a pal-function.. it seemed to be missing from your code..
# Create a continuous palette function
pal <- colorNumeric( palette = "Reds", domain = parcels$num.parcels )
#calculate fillopacity
labels <- sprintf(
"<strong>%s County</strong><br/>
Parcels: %g<br/>",
parcels$county, parcels$num.parcels
) %>% lapply(htmltools::HTML)
leaflet(parcels) %>%
setView(-119, 37.9, 6) %>%
addTiles() %>%
addPolygons(
fillColor = ~pal(num.parcels),
weight = 2,
opacity = 1,
color = 'black',
dashArray = '2',
fillOpacity = ~num.parcels / max(num.parcels),
highlightOptions = highlightOptions(color = "red", weight = 3,
bringToFront = TRUE),
label = labels,
labelOptions = labelOptions(
style = list("font-weight" = "normal", padding = "4px 8px"),
textsize = "15px",
direction = 'auto')) %>%
addLegend(pal = pal, values = ~num.parcels, opacity = 1, title = "Number of Parcels",
position = "bottomleft")