title: How can I use name of a month in x-axis in ggplot2
body: <p>I have tried to plot months on the x axis using <code>ggplot2</code>, but the month names are automatically shown as numeric numbers with decimals. How could I force the script to plot month names insted of numbers? I used this code: </p>
  
  <pre><code>ggplot(df3, aes(x = month, y = PM)) + 
  geom_line(aes(col = factor(travel))) + 
  xlab("Month") + 
  ylab(expression(paste("PM(",mu,"g/", m^3,")", sep=""))) 
  </code></pre>
  
  <p>Data is found below:</p>
  
  <pre><code>df3 &lt;- structure(list(month = c(9, 10, 9, 10, 1, 2, 3, 4, 5, 9, 10, 
                              1, 2, 3, 4, 5, 9, 10, 11, 12, 1, 2, 3, 4, 5, 11), 
                    travel = c("Diesel bus", 
                               "Diesel bus", "Diesel bus", "Diesel bus", "Diesel bus", "Diesel car", 
                               "Diesel car", "Diesel car", "Diesel car", "Diesel car", "Bicycle", 
                               "Bicycle", "Bicycle", "Bicycle", "Bicycle", "Gasoline car", "Gasoline car", 
                               "Gasoline car", "Gasoline car", "Gasoline car", "Elcectric bus", 
                               "Elcectric bus", "Elcectric bus", "Elcectric bus", "Elcectric bus", 
                               "Elcectric bus"), 
                    PM = c(22.6496512922918, 18.1829352554303, 
                           28.776408085308, 30.1441935430254, 23.8938954711914, 21.8288997171693, 
                           22.7177263732526, 29.8606175809457, 30.530998468399, 30.1288699182287, 
                           28.4038889338888, 19.4761033463478, 18.9449487406838, 20.3568456145256, 
                           16.5431814479828, 12.8668955993652, 21.6255497367427, 21.8725590587368, 
                           14.7631275227865, 12.5790810203552, 15.1794028663635, 19.3508881492176, 
                           15.895525373979, 15.3945024820964, 15.5982689292758, 12.1219868087769
                    )),
               .Names = c("month", "travel", "PM"),
               row.names = c(NA, -26L),
               class = "data.frame")
  </code></pre>