在ggplot2中向绘图和图例添加水平线

Question

这段代码创建了一个漂亮的图，但是我想在y = 50处添加一条水平黑线，并让图例在图例中显示一条带有文本“ cutoff”的黑线，​​但在图例中保留源代码。 我可以使用geom_line添加该行，但是无法在图例中获得该行。

    library(ggplot2)
    the.data <- read.table( header=TRUE, sep=",", 
    text="source,year,value
    S1,1976,56.98
    S1,1977,55.26
    S1,1978,68.83
    S1,1979,59.70
    S1,1980,57.58
    S1,1981,61.54
    S1,1982,48.65
    S1,1983,53.45
    S1,1984,45.95
    S1,1985,51.95
    S1,1986,51.85
    S1,1987,54.55
    S1,1988,51.61
    S1,1989,52.24
    S1,1990,49.28
    S1,1991,57.33
    S1,1992,51.28
    S1,1993,55.07
    S1,1994,50.88
    S2,1993,54.90
    S2,1994,51.20
    S2,1995,52.10
    S2,1996,51.40
    S3,2002,57.95
    S3,2003,47.95
    S3,2004,48.15
    S3,2005,37.80
    S3,2006,56.96
    S3,2007,48.91
    S3,2008,44.00
    S3,2009,45.35
    S3,2010,49.40
    S3,2011,51.19") 
    ggplot(the.data, aes( x = year, y = value ) ) + 
        geom_point(aes(colour = source)) + 
        geom_smooth(aes(group = 1))

Answer 1

（1）试试这个：

cutoff <- data.frame( x = c(-Inf, Inf), y = 50, cutoff = factor(50) )
ggplot(the.data, aes( year, value ) ) + 
        geom_point(aes( colour = source )) + 
        geom_smooth(aes( group = 1 )) + 
        geom_line(aes( x, y, linetype = cutoff ), cutoff)

（2）关于您的评论，如果您不希望将截止值列为单独的图例，那么只需在图上标注截止值线会更容易：

ggplot(the.data, aes( year, value ) ) + 
    geom_point(aes( colour = source )) + 
    geom_smooth(aes( group = 1 )) + 
    geom_hline(yintercept = 50) + 
    annotate("text", min(the.data$year), 50, vjust = -1, label = "Cutoff")

更新资料

这似乎更好，并且可以概括为如下所示的多行代码：

line.data <- data.frame(yintercept = c(50, 60), Lines = c("lower", "upper"))
ggplot(the.data, aes( year, value ) ) + 
        geom_point(aes( colour = source )) + 
        geom_smooth(aes( group = 1 )) + 
        geom_hline(aes(yintercept = yintercept, linetype = Lines), line.data)

Answer 2

另一个解决方案：

gg <- ggplot(the.data, aes( x = year, y = value ) ) + 
        geom_point(aes(colour = source)) + 
        geom_smooth(aes(group = 1))

cutoff <- data.frame(yintercept=50, cutoff=factor(50))
gg + 
  geom_hline(aes(yintercept=yintercept, linetype=cutoff), data=cutoff, show_guide=TRUE) 

此代码生成与@G点（1）完全相同的图形。 格洛腾迪克。 但是，更容易适应多层图形。