title: tmap pch shape and color tm_symbol
body: <p>I am using tmap to make a map that will have both shape and color aesthetics. However, I am noticing strange behavior in my numeric scale for color (it is producing scales of 0 min to 10 min, 10 min to 20 min, etc.). Why is tmap returning this scale? I would like to have control over the pch of the symbols, so that A and B would specified to pch's 21 and 19 respectively, how can this be accomplished?</p>
  
  <p>My example:</p>
  
  <pre><code>library(tmap) 
  data(metro)
  metro$new_var &lt;- c("A","B")
  metro$new_var &lt;- factor(metro$new_var)
  tm_shape(metro) +
    tm_symbols(shape="new_var", col = "pop2010") +
    tm_layout(legend.outside = TRUE, legend.outside.position = "bottom", legend.stack = "horizontal")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/aQkDA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aQkDA.png" alt="enter image description here"></a></p>