title: us.cities from maps package in r not working
body: <p>I am trying to add US cities to my map in R.</p>
  
  <p>When I add world cities: </p>
  
  <pre><code>ggplot(data = usa) +
    geom_polygon(aes(x = longitude, y = latitude, group = group, fill = id)) +
    coord_quickmap() + 
    guides(fill = FALSE) +
    theme(axis.text = element_blank()) +
    theme(axis.ticks = element_blank()) +
    theme(axis.title = element_blank()) +
    xlim(-169, -67) +
    ylim(17,72)+
    scale_fill_discrete()+
    geom_point(data=world.cities, aes(x=long, y = lat, size= capital))
  </code></pre>
  
  <p>or canada cities:</p>
  
  <pre><code>ggplot(data = usa) +
    geom_polygon(aes(x = longitude, y = latitude, group = group, fill = id)) +
    coord_quickmap() + 
    guides(fill = FALSE) +
    theme(axis.text = element_blank()) +
    theme(axis.ticks = element_blank()) +
    theme(axis.title = element_blank()) +
    xlim(-169, -67) +
    ylim(17,72)+
    scale_fill_discrete()+
    geom_point(data=canada.cities, aes(x=long, y = lat, size= capital))
  </code></pre>
  
  <p>it works just fine. However, when I try replacing it with <code>us.cities</code>, which is supposed to be part of the <code>maps</code> package (<a href="https://cran.r-project.org/web/packages/maps/maps.pdf" rel="nofollow noreferrer">https://cran.r-project.org/web/packages/maps/maps.pdf</a>), it gives the error: </p>
  
  <blockquote>
    <p>ggplot2 doesn't know how to deal with data of class character.</p>
  </blockquote>
  
  <p>any ideas on what could be the issue??</p>
  
  <p>edit: more code:</p>
  
  <pre><code>library(maps)
  library(fiftystater)
  
  usa &lt;- data("fifty_states")
  us.cities &lt;- data(us.cities)
  usa.cities &lt;- world.cities %&gt;% filter(country.etc=="USA")
  map &lt;- map_data("world")
  hawaii &lt;- read.csv("hawaii.csv", stringsAsFactors = FALSE)
  alaska &lt;- read.csv("alaska.csv", stringsAsFactors = FALSE)
  forty8states &lt;- fifty_states %&gt;% filter(id != "hawaii") %&gt;% filter(id    !="alaska") %&gt;% select(long, lat, id, group)
  
  alaska &lt;- read.csv("alaska.csv", stringsAsFactors = FALSE)
  hawaii &lt;- read.csv("hawaii.csv", stringsAsFactors = FALSE)
  
  usa &lt;- rbind(forty8states, alaska, hawaii)
  usa &lt;- usa %&gt;% mutate(longitude = round(long, digits = 2)) 
  usa &lt;- usa %&gt;% mutate(latitude = round(lat, digits = 2))
  </code></pre>
  
  <p>the hawaii and alaska csv's are the hawaii and alaska data from the <code>world</code> data frame from <code>maps</code>, with some editing so they're in the same format as the rest.</p>