title: Remove countries' political borders from ggplot2 map
body: <p>I need to remove the political borders of the countries from the following <code>ggplot2</code> map:</p>
  
  <pre><code>library(ggplot2)
  
  world = map_data('world')
  
  plot=ggplot() +
  geom_polygon(data=world, aes(x=long, y=lat, group=group), fill='NA', color='black', size=0.2)
  
  print(plot)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/SAxLc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SAxLc.png" alt="enter image description here"></a></p>
  
  <p>Any suggestion on how I can do this?
  Thanks</p>