title: Draw Boundary by zip code and create a heat map
body: <p>I need to create heat map with 3 digit zip boundary.
  I have 3 digit zip and count data like this</p>
  
  <pre><code>zip &lt;- c(790, 791, 792, 793)
  count &lt;- c(0, 100, 20, 30)
  TX &lt;- data.frame(zip, count)
  </code></pre>
  
  <p>Also, I draw TX map.</p>
  
  <pre><code>library(ggplot2)
  library(ggmap)
  library(maps)
  library(mapdata)
  states &lt;- map_data("state")
  texas&lt;- subset(states, region =="texas")
  
  ggplot(data = texas) + 
  geom_polygon(aes(x = long, y = lat), fill = "gray", color = "black") 
  </code></pre>
  
  <p>What I want to achieve is to (1) draw boundary with 3 digit zip code and (2) create the heat map using count column. The outcome will looks like this with heat map coloring.  </p>
  
  <p><a href="https://i.stack.imgur.com/agFBy.gif" rel="noreferrer"><img src="https://i.stack.imgur.com/agFBy.gif" alt="enter image description here"></a></p>