title: Error : don't know how to add ggplot to a plot
body: <p>I would like to plot a map with points of differents colors for each actor
  which commit violence.</p>
  
  <p>Thanks you for your help :)</p>
  
  <pre><code>library("readxl", lib.loc = "C:\Users\Student\Downloads\sll-ledsierre-   leonelocalsource1991-2001.xlsx")
  library("ggplot2")
  library("ggmap")
  
  qmap("Sierraleone" , zoom = 8)
  
  df &lt;- read_xlsx("sll-ledsierre-leonelocalsource1991-2001.xlsx")
  
  map &lt;- qmap("Sierraleone" , zoom = 8)
  #only violence against civilians
  
  df1&lt;- df
  
  install.packages("dplyr")
  library(dplyr)
  install.packages("magrittr")
  library(magrittr)
  
  df1 &lt;- filter(df1, df1$EVENT_TYPE == "Violence against civilians")
  
  map + ggplot(data = df1, aes(x = LONGITUDE, y = LATITUDE)) +
  geom_point(alpha = 0,5, size = 1) +
  aes(colour=ACTOR1)
  </code></pre>
  
  <p>an example of my dataset <code>(head(df))</code> with only the interesting columns for my map</p>
  
  <pre><code>ACTOR1     LATITUDE     LONGITUDE
  
  1 RUF: Revolutionary United Front     8.11     -11.6
  2 RUF: Revolutionary United Front     8.11     -11.6
  3 RUF: Revolutionary United Front     7.96     -11.7
  4 RUF: Revolutionary United Front     7.96     -11.7
  5 RUF: Revolutionary United Front     7.96     -11.7
  6 RUF: Revolutionary United Front     7.96     -11.7
  </code></pre>
  
  <p>I get this error message </p>
  
  <p>Error: Don't know how to add <code>ggplot(data = df1, aes(x = LONGITUDE, y = LATITUDE))</code> to a plot</p>