docs: move comments to prose

vignettes/articles/extending.Rmd

@@ -43,38 +43,44 @@ Often when passing a list from R to JavaScript it is desirable to remove any nul
 
 # Example
 
-Here is a small example which shows how you can integrate heatmap functionality using a [plugin](https://leaflet.github.io/Leaflet.heat/).
+Here is a small example which shows how you can integrate heatmap functionality using a [plugin](https://leaflet.github.io/Leaflet.heat/). 
+
+There are four key steps to add this plugin to a Leaflet map in R:
+
+1. Define a function that takes a `htmltools::htmlDependency()` object and adds it to the map. This ensures that however or whenever the map gets rendered, the plugin will be loaded into the browser.
+
+2. Actually create the `htmltools::htmlDependency()`. This function tells the browser our plugin name, version, and where to find the script. There's also a stylesheet argument if the plugin comes with CSS files.
+
+3. Use our `registerPlugin()` function to register the plugin dependency with the `leaflet()` object. As the first argument of our function is `map`, this can be achieved as part of a pipeline.
+
+4. We need to tell `Leaflet` what to actually *do* with our new plugin using `htmlwidgets::onRender()`. This adds additional rendering logic to this specific widget, written in JavaScript. Note that, in the below example, `this` refers to the `Leaflet` (JS) map object itself.
 
 ```{r, fig.height=4}
 library(leaflet)
 library(htmltools)
 library(htmlwidgets)
 
-# This tells htmlwidgets about our plugin name, version, and
-# where to find the script. (There's also a stylesheet argument
-# if the plugin comes with CSS files.)
+# function to register plugin
+registerPlugin <- function(map, plugin) {
+  map$dependencies <- c(map$dependencies, list(plugin))
+  map
+}
+
+# define plugin as a HTML dependency
 heatPlugin <- htmlDependency(
   "Leaflet.heat",
   "0.2.0",
   src = c(href = "http://leaflet.github.io/Leaflet.heat/dist/"),
   script = "leaflet-heat.js"
 )
 
-# A function that takes a plugin htmlDependency object and adds
-# it to the map. This ensures that however or whenever the map
-# gets rendered, the plugin will be loaded into the browser.
-registerPlugin <- function(map, plugin) {
-  map$dependencies <- c(map$dependencies, list(plugin))
-  map
-}
-
 # initialise leaflet map in R
 leaflet() %>%
   addTiles() %>%
   fitBounds(min(quakes$long), min(quakes$lat), max(quakes$long), max(quakes$lat)) %>%
   # Register heatmap plugin on this map instance
   registerPlugin(heatPlugin) %>%
-  # Add custom JS logic; `this` refers to the Leaflet (JS) map object.
+  # Add custom JS logic
   onRender("function(el, x, data) {
     data = HTMLWidgets.dataframeToD3(data);
     data = data.map(function(val) { return [val.lat, val.long, val.mag*100]; });