title: tmap coming up with blank map with one variable (but values are there)
body: <p>I have two numeric variables I'm using to create two thematic maps using the tmap package. The same code works for one of the variables, but creates a blank map for the other. I created a small subset of the data to reproduce the error, here the last category still has some colour, but the rest are blank. In the full data set all categories are blank. </p>
  
  <p>I really have no idea what is happening, the two variables are same class/type, and I get a proper plot using plot(), so I cannot think what's wrong! Any tips would be much appreciated!</p>
  
  <p>Here is the code I tried: </p>
  
  <pre><code>library(sf)
  library(tmap)
  
  
  df &lt;- dget("https://www.dropbox.com/s/8yap9bdknyvelwq/tmap_df.txt?dl=1") 
  
  #works:
  tm_shape(df) + 
    tm_fill("v1", style = "quantile", title = "Change in V1", border.alpha = 0, palette = "-RdBu", midpoint = 0)
  
  #doesn't work:
  tm_shape(df) + 
    tm_fill("v2", title = "Change in V2",  style = "quantile", border.alpha = 0, palette = "-RdBu", midpoint = 0)
  
  #the two variables are the same class/type
  class(df$v1) == class(df$v2)
  typeof(df$v1) == typeof(df$v2)
  
  #if I use plot(), it works
  plot(df[,1])
  plot(df[,2])
  
  </code></pre>
  
  <p>Any ideas?</p>