title: How to add lines of longitude and latitude on a map using ggplot2?
body: <p>I am now plotting the map of Canada using <code>ggplot2</code>. Because the default projection method is "aea"(Albers Equal Area), so the longitude and latitude are straight lines on the map.  I wonder how I can display the longitude and latitude in the form of "110W, 100W, 90W" and "50N, 60N, 70N" on the map. They should be curves. Thanks a lot.</p>
  
  <p>The arcgis shapfile is downloaded from <a href="https://www.arcgis.com/home/item.html?id=dcbcdf86939548af81efbd2d732336db" rel="nofollow noreferrer">https://www.arcgis.com/home/item.html?id=dcbcdf86939548af81efbd2d732336db</a>
  <a href="https://i.stack.imgur.com/Gakst.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Gakst.png" alt="enter image description here"></a></p>
  
  <pre><code>library(ggplot2)
  library(rgdal)
  countries&lt;-readOGR("Canada.shp", layer="Canada")
  ggplot()+geom_polygon(data=countries,aes(x=long,y=lat,group=group),fill='white',color = "black")
  </code></pre>
  
  <p>The final result should be like this.
  <a href="https://i.stack.imgur.com/Kh2le.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Kh2le.jpg" alt="enter image description here"></a></p>