x轴为mm-dd时使用ggplot2编辑轴刻度标签

Question

I have a dataset where the x-axis is a date, but it is only mm-dd (no year). I am using year as a group variable as I am trying to show a YOY change on the same plot. The x-axis labeling is too crowded and I'd like to limit the tick mark labels so that not every date is shown. This could be every other day, every third day, one day a week -- any of these would work.

I have tried a few solutions but cannot get them to work, I'm assuming because my x-axis is not a Date, but a character. (Previous to arriving at this mm-dd solution for the x-axis, I tried plotting the x-axis with a yyyy-mm-dd Date format, but was unsuccessful in figuring out how to get ggplot2 to ignore the "yyyy" part.)

An example:

myDF <- data.frame(
        myDate = format(seq(as.Date("2014-02-01"), 
                 length=28, by="1 day"), "%m-%d"),
        myVar = sample(100,28),
        myGroup = sample(2,28,TRUE)
                   )
head(myDF)

  myDate myVar myGroup
  02-01    87       1
  02-02    34       1
  02-03    48       2
  02-04    59       1
  02-05    98       1
  02-06    18       2

ggplot(myDF, aes(myDate, myVar, group=myGroup, color=as.factor(myGroup))) + 
geom_line() 

I have tried:

ggplot(myDF, aes(myDate, myVar, group=myGroup, color=as.factor(myGroup))) +   
geom_line() + scale_x_discrete(breaks = c(1,10,20))

This appears to confuse ggplot since the labels disappear completely. (Same result with a seq() attempt.)

I have also tried:

ggplot(myDF, aes(myDate, myVar, group=myGroup, color=as.factor(myGroup))) + 
geom_line() + scale_x_date(breaks = "1 week")

This throws an error re: myDate not being a Date.

I've already switched the format of the tick labels to be vertical, but it is still too crowded on the plot.

Any tips would be very much appreciated. Thanks!

Answer 1

If you want to use myDate variable without the year (as character) then one solution would be to use scale_x_discrete() and then provide myDF$myDate as breaks= argument and select sequence of values you want to show. In this example I selected every 7th value.

ggplot(myDF, aes(myDate, myVar, group=myGroup, color=as.factor(myGroup))) +   
      geom_line() + scale_x_discrete(breaks = unique(myDF$myDate)[seq(1,28,7)])