r - 消除ggplot折线图中的X轴间隙

当我使用ggplot创建折线图时，x轴的两侧会出现两个间隙，如下所示：

如何防止这种情况，使直线在x轴的两个边缘开始和结束，而不是仅仅在之前/之后？

这是我到目前为止的代码：

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

ggplot(data=combined, aes(x = Date, y = Spread)) + geom_line()