USGS - science for a changing world

Using Leaflet and htmlwidgets in a Hugo blog post

Laura DeCicco

We are excited to use many of the JavaScript data visualizations in R using the htmlwidgets package in future blog posts. Having decided on using Hugo, one of our first tasks was to figure out a fairly straightforward way to incorporate these widgets. This post describes the basic process to get a basic leaflet map in our Hugo-generated blog post.

In this example, we are looking for phosphorus measured throughout Wisconsin with the dataRetrieval package. Using dplyr, we filter the data to sites that have records longer than 15 years, and more than 50 measurements.

library(dataRetrieval)
pCode <- c("00665")
phos.wi <- readNWISdata(stateCd="WI", parameterCd=pCode,
                     service="site", seriesCatalogOutput=TRUE)

library(dplyr)
phos.wi <- filter(phos.wi, parm_cd %in% pCode) %>%
            filter(count_nu > 50) %>%
            mutate(period = as.Date(end_date) - as.Date(begin_date)) %>%
            filter(period > 15*365)

Plot the sites on a map:

library(leaflet)
leafMap <- leaflet(data=phos.wi) %>% 
  addProviderTiles("CartoDB.Positron") %>%
  addCircleMarkers(~dec_long_va,~dec_lat_va,
                   color = "red", radius=3, stroke=FALSE,
                   fillOpacity = 0.8, opacity = 0.8,
                   popup=~station_nm)

Next, we use the htmlwidgets package to save a self-contained html file. The following code could be generally hid from the reader using echo=FALSE.

library(htmlwidgets)
library(htmltools)

currentWD <- getwd()
dir.create("static/leaflet", showWarnings = FALSE)
setwd("static/leaflet")
saveWidget(leafMap, "leafMap.html")
setwd(currentWD)

The html that was saved with the saveWidget function can be called with the iframe html tag.

<iframe seamless src="/static/leaflet/leafMap/index.html" width="100%" height="500"></iframe>

When building the site, Hugo converts the “leafMap.html” to “leafMap/index.html”.

One issue for our Hugo theme was that the created widget page is included in the overall blog index. The was fixed by adding a line to the overall index.html layout page to only lists pages with dates above Jan. 1, 1970 (so really, any legitimate date):

{{ if ge $value.Date.Unix 0 }}
  <div class="col-sm-4">
    {{ .Render "grid" }}
  </div>
{{ end }}

A screenshot of the map was taken to use for the thumbnail in the blog index.

Questions

Please direct any questions or comments on dataRetrieval to: https://github.com/USGS-R/dataRetrieval/issues