title: Points in a map with qmplot
body: <p>I have to plot some points in a map. I want to use <code>qmplot</code>, because it seems simple and effective.</p>
  
  <p>I have followed <a href="https://rpubs.com/jiayiliu/ggmap_examples" rel="nofollow">this tutorial</a>, but I cannot get it to work:</p>
  
  <pre><code># Libraries
  install.packages("maps")
  install.packages("ggmap")
  library(maps)
  library(ggmap)
  
  # Loading European map:     
  map &lt;- get_map(location = 'Europe', zoom = 4)
  ggmap(map)
  
  # Madrid coordinates
  df &lt;- data.frame(lon=c(-3.757324), lat=c(40.441721))
  
  # Plotting the point
  qmplot(df$lon, df$lat)
  qmplot(df$lon, df$lat, data = df)
  </code></pre>
  
  <p>I get this error:</p>
  
  <blockquote>
    <p>Error in &#96;[.data.frame&#96;(data, , deparse(substitute(x))) :<br>
    &nbsp;&nbsp;undefined columns selected</p>
  </blockquote>