ggplot: Multiple years on same plot by month

To get a separate line for each year, you need to extract the year from each date and map it to colour. To get months (without year) on the x-axis, you need to extract the month from each date and map to the x-axis.

library(zoo)
library(lubridate)
library(ggplot2)

Let's create some fake data with the dates in as.yearmon format. I'll create two separate data frames so as to match what you describe in your question:

# Fake data
set.seed(49)
dat1 = data.frame(date = seq(as.Date("2015-01-15"), as.Date("2015-12-15"), "1 month"),
                 value = cumsum(rnorm(12)))
dat1$date = as.yearmon(dat1$date)

dat2 = data.frame(date = seq(as.Date("2016-01-15"), as.Date("2016-12-15"), "1 month"),
                  value = cumsum(rnorm(12)))
dat2$date = as.yearmon(dat2$date)

Now for the plot. We'll extract the year and month from date with the year and month functions, respectively, from the lubridate package. We'll also turn the year into a factor, so that ggplot will use a categorical color palette for year, rather than a continuous color gradient:

ggplot(rbind(dat1,dat2), aes(month(date, label=TRUE, abbr=TRUE), 
                value, group=factor(year(date)), colour=factor(year(date)))) +
  geom_line() +
  geom_point() +
  labs(x="Month", colour="Year") +
  theme_classic()

month value year
Jan 99.99 2015
Feb 99.90 2015
Jan 100 2016
Feb 99.95 2016

You need one longform dataset that has a year column. Then you can plot both lines with ggplot

ggplot(dataset, aes(x = month, y = value, color = year)) + geom_line()