Changing Date Labels From Odd to Even Years - r

Question

I want to make a seemingly trivial adjustment to the chart pictured below:
I would like the labels along the x-axis to be even years, rather than odd years. So instead of going from 2009 -> 2011 -> 2013, they should go from 2008 -> 2010 -> 2012, and so forth...
How do I go about doing this?
Here is the code:
germany_yields <- read.csv(file = "Germany 10-Year Yield Weekly (2007-2020).csv", stringsAsFactors = F)
italy_yields <- read.csv(file = "Italy 10-Year Yield Weekly (2007-2020).csv", stringsAsFactors = F)
germany_yields <- germany_yields[, -(3:6)]
italy_yields <- italy_yields[, -(3:6)]
colnames(germany_yields)[1] <- "Date"
colnames(germany_yields)[2] <- "Germany.Yield"
colnames(italy_yields)[1] <- "Date"
colnames(italy_yields)[2] <- "Italy.Yield"
combined <- join(germany_yields, italy_yields, by = "Date")
combined <- na.omit(combined)
combined$Date <- as.Date(combined$Date,format = "%B %d, %Y")
combined["Spread"] <- combined$Italy.Yield - combined$Germany.Yield
fl_dates <- c(tail(combined$Date, n=1), head(combined$Date, n=1))
ggplot(data=combined, aes(x = Date, y = Spread)) + geom_line() +
scale_x_date(limits = fl_dates,
expand = c(0, 0),
date_breaks = "2 years",
date_labels = "%Y")

Answer 1

A -- not very elegant -- way would be to put these arguments in your scale_x_date() :
scale_x_date(date_labels = "%Y",
breaks = ymd(unique(year(combined$fl_dates)[year(combined$fl_dates)%%2 == 0]), truncated = 2L)
(we define breaks manually, by subsetting the whole range of dates and keeping the even years)

Answer 2

That's actually fairly simple. Just set the lower limit to an even number, and set the upper limit to NA. As you haven't provided a reproducible example, here on some fake data.
library(tidyverse)
mydates <- seq(as.Date("2007/1/1"), by = "3 months", length.out =100)
df <- tibble(
myvalue = rnorm(length(mydates))
)
# without limits argument
ggplot(df ) +
aes(x = mydates, y = myvalue) +
geom_line(size = 1L, colour = "#0c4c8a") +
scale_x_date(date_breaks = "2 years",
date_labels = "%Y")
# with limits argument
ggplot(df ) +
aes(x = mydates, y = myvalue) +
geom_line(size = 1L, colour = "#0c4c8a") +
scale_x_date(date_breaks = "2 years",
date_labels = "%Y",
limits = c(as.Date("2006/1/1"), NA))
Created on 2020-04-29 by the reprex package (v0.3.0)