title: R - Plot with tmap() does not display the legend
body: <p>I use the <code>tmap</code> package. I plot a map with the <code>qtm()</code> function but the legend is not displayed. Indeed, I understand that the legend is automatically display, unless you specify to hide it.</p>
  
  <p>I also try to use a work-around with the <code>tm_add_legend()</code> but it doesn't work.</p>
  
  <p><a href="https://i.stack.imgur.com/CvAvk.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CvAvk.jpg" alt="MyPlot"></a></p>
  
  <p>Here is my code :</p>
  
  <pre><code>library(sp) ; library(rgdal) ; library(rgeos) ;
  library(tmap) ; library(maptools)
  
  Directory &lt;- ""
  
  MapFrance &lt;- readOGR(dsn = paste0(Directory, "Departements"),  layer = "DEPARTEMENT")
  
  MapFrance@data$Score &lt;- round(runif(96, min = 1, max = 10), 0)
  
  brk &lt;- c(1, 2.5, 5, 7.5, 10)
  ClassScore &lt;- classIntervals(MapFrance@data$Score, style = "fixed", fixedBreaks = brk)
  Palette &lt;- brewer.pal(length(brk) - 1, "Greens")
  
  MapFrance@data$Categorie &lt;- as.character(cut(MapFrance@data$Score,
                                               breaks = ClassScore$brks,
                                               labels = Palette,
                                               include.lowest = TRUE))
  
  ExampleData &lt;- MapFrance[MapFrance$CODE_REG == "11", ]
  
  qtm(ExampleData, fill = "Categorie", text = "CODE_DEPT", text.size = "AREA", style = "gray",
      text.root = 5, fill.title = "Opinion scale")
  </code></pre>
  
  <p>And a link to the geographic data :
  <a href="https://drive.google.com/file/d/1wksMnkufQPdOfSbhWoZbPKMqlcLcll_j" rel="nofollow noreferrer">https://drive.google.com/file/d/1wksMnkufQPdOfSbhWoZbPKMqlcLcll_j</a></p>