title: How to "addTiles" on top of "addPolygons" in R's Leaflet?
body: <p>How do you set the layer order in R's leaflet package so that tiles show up on top of polygons filled with color?</p>
  
  <p>Here's what I've got so far:</p>
  
  <pre><code>require(leaflet)
  require(acs)
  require(tigris)
  require(rgdal)
  census.income.end.year = 2015
  county = 17
  nd.counties=acs.fetch(geography=geo.make(state="ND", county=county), 
                        table.number="B01003", endyear = 2015)
  
  tracts &lt;- tigris::tracts(state = 'ND', county = county, cb=FALSE, year = 2015)
  
  # create a geographic set to grab tabular data (acs)
  geo&lt;-geo.make(state=c("ND"),
                county = county,
                tract="*")
  
  # add in median income
  median.income &lt;- acs.fetch(endyear = census.income.end.year, 
                             geography = geo, 
                             variable = c("B19013_001"))
  income_df &lt;- data.frame(paste0(as.character(median.income@geography$state), 
                                 str_pad(as.character(median.income@geography$county), 3, 'left', '0'), 
                                 str_pad(as.character(median.income@geography$tract), 5, 'left', '0')), 
                          median.income@estimate)
  
  rownames(income_df)&lt;-1:nrow(income_df)
  names(income_df)&lt;-c("GEOID", "hhincome")
  income_merged &lt;- geo_join(tracts, income_df, "GEOID", "GEOID")
  
  income_merged &lt;- spTransform(income_merged, CRS("+init=epsg:4326"))
  
  qpal &lt;- colorQuantile("plasma", income_df$hhincome, n = 4)
  
  leaflet() %&gt;% 
    setView( -96.7898, 46.8772, zoom=11) %&gt;% 
    addPolygons(data = income_merged, 
                fillColor = qpal(income_merged$hhincome), 
                fillOpacity = 1, 
                weight = 0.3) %&gt;%
    addProviderTiles(providers$Hydda.RoadsAndLabels) 
  </code></pre>
  
  <p>Ultimately, I'ld like to do this with addTiles (instead of addProviderTiles as in the above code) using a custom MapBox, but I can't figure out how to make that reproducible for this example... given that you need a key to access custom MapBox tiles (BTW, I've created a custom MapBox tile that should be transparent except for roads and labels, so the underlying polygons should "show thru.")</p>
  
  <p>Here is one way to do add a tile on top of a circle with the non-R version of leaflet: <a href="http://jsfiddle.net/dcu9pz2w/" rel="noreferrer">http://jsfiddle.net/dcu9pz2w/</a>, but I don't see how to make that work in my context. I think adding "panes" may be the way to go, but I don't see that functionality in R leaflet. Also, I explored z-index values, but that seemed to be a dead end.</p>
  
  <p>Any help is much appreciated!</p>