r - map dates from multiple years to a single year

Question

I'd like to create a time series chart in R that compares this year to last year. I'd like to use this year's dates as my x axis.

What is the function that, given a set of dates from 2 or more years, will make them all the same year?

Data example:

mydata <- data.frame(
    date.col = as.Date(c('2014-05-23', '2014-05-24', '2014-05-25', '2015-05-23', '2015-05-24', '2015-05-25', '2016-05-23', '2016-05-24', '2016-05-25')),
    value = c(10,23,15,13,26,17,22,30,19))

    date.col value
1 2014-05-23    10
2 2014-05-24    23
3 2014-05-25    15
4 2015-05-23    13
5 2015-05-24    26
6 2015-05-25    17
7 2016-05-23    22
8 2016-05-24    30
9 2016-05-25    19

Expected output:

    date.col value   adj.date
1 2014-05-23    10 2016-05-23
2 2014-05-24    23 2016-05-24
3 2014-05-25    15 2016-05-25
4 2015-05-23    13 2016-05-23
5 2015-05-24    26 2016-05-24
6 2015-05-25    17 2016-05-25
7 2016-05-23    22 2016-05-23
8 2016-05-24    30 2016-05-24
9 2016-05-25    19 2016-05-25

Ideally, the function could fit easily into a dplyr chain:

mydata %>% normalize.dates(date.col, fit.to='2016') %>% ggplot(...)

Answer 1

This works well:

as.Date(format(date.col, '2016-%m-%d'))

Fit into a dplyr chain:

my.data %>% mutate(adj.date = as.Date(format(date.col, '2016-%m-%d'))) %>%
    ggplot(aes(x=adj.date)) + ...

Useful for time series plots when all dates need to be normalized for year over year comparison:

mydata %>%
    mutate(adj.date = as.Date(format(date.col, '2016-%m-%d'))) %>%
    ggplot(aes(x=adj.date, y=value, color=as.factor(year(date.col)))) + geom_line()