How can I use name of a month in x-axis in ggplot2

Question

I have tried to plot months on the x axis using ggplot2, 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:

ggplot(df3, aes(x = month, y = PM)) + 
geom_line(aes(col = factor(travel))) + 
xlab("Month") + 
ylab(expression(paste("PM(",mu,"g/", m^3,")", sep=""))) 

Data is found below:

df3 <- 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")

Answer 1

you can also use scale_x_discrete

ggplot(df3, aes(x=month, y=PM)) +
    geom_line(aes(col = factor(travel)))+
    xlab("Month")+ ylab (expression(paste("PM(",mu, "g/", m^3,")", sep=""))) +
    scale_x_discrete(labels=month.abb)    

Answer 2

Alternative solution with the scales package

First create fake full-dates and then just plot the month without year and date:

library(scales)
df3$yearmonth <- as.Date(paste0("2018-", df3$month, "-1"))

# with month names (latin characters)
ggplot(df3, aes(x = yearmonth, y = PM, color = as.factor(travel))) +
  geom_line() + 
  xlab("Month")+ ylab (expression(paste("PM(",mu, "g/", m^3,")", sep=""))) +
  scale_x_date(labels = date_format("%b"))

# with month numbers
ggplot(df3, aes(x = yearmonth, y = PM, color = as.factor(travel))) +
  geom_line() +
  xlab("Month")+ ylab (expression(paste("PM(",mu, "g/", m^3,")", sep=""))) +
  scale_x_date(labels = date_format("%m"))