title: How to manipulate tmap legend?
body: <p>I'm creating a thematic map of percent change per year for bird species. here is the code I have:</p>
  
  <pre><code>tm_shape(grid83)+
    tm_fill("trend", title = "Percent change per Year", textNA = "None counted", style="fixed",
      breaks=c(-Inf, -1.5, -0.25, 0.25, 1.5, Inf),
      palette = c("red", "orange", "yellow", "turquoise", "blue", "white"))+
    tm_borders(NA)+
  tm_shape(uscan83)+ # add US and CAN 
    tm_borders()+
  tm_layout(
    "Western Grebe",
    legend.title.size=1,
    legend.text.size = 0.6,
    legend.position = c("left","bottom"),
    legend.bg.color = "white",
    legend.digits = 5,
    legend.bg.alpha = 1)
  </code></pre>
  
  <p>Currently all NA values show up as gray. I've tried to change the color palette:</p>
  
  <pre><code>palette = c("red", "orange", "yellow", "turquoise", "blue", "white"))
  </code></pre>
  
  <p>but this doesn't seem to be working. NA values are all still gray. What am I doing wrong?</p>
  
  <p>Thanks so much!</p>