如何在 ggplot2 中的 x 轴上使用月份名称

我尝试使用以下方法在 x 轴上绘制月份ggplot2，但月份名称会自动显示为带小数的数字。我如何强制脚本绘制由数字组成的月份名称？我使用了这段代码：

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

查到的数据如下：

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")

你也可以使用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)