r - geom_rect 失败 : Error in eval(expr, 环境，enclos):找不到对象 'variable'

Question

我试图复制 ggplot2 书第 86 页上 Hadley Wickham 的例子，在那里他覆盖了 geom_rects，显示执政党对失业趋势的影响。这是我的核心代码，包含一些示例数据:

library("ggplot2")
library("reshape2")
library("lubridate")

con <- textConnection("Date,Var1,Var2
8-Jan-12,100.8,116
15-Jan-12,99.4,115.5
22-Jan-12,98.4,115
28-Jan-12,97.1,114
4-Feb-12,95.9,112
11-Feb-12,95.3,113
18-Feb-12,93.9,111.5
25-Feb-12,93.5,111.5
3-Mar-12,92.6,110
10-Mar-12,91.4,108
17-Mar-12,90.2,106.8
24-Mar-12,90,107.5
31-Mar-12,89.9,106
5-Apr-12,90.4,106.5
12-Apr-12,89.8,106") 

track <- read.csv(con, header=TRUE)
track$Date <- as.Date( dmy(track$Date) )

track <- melt(track, 
            id.vars = c("Date"))

con <- textConnection("City,From,To
Auckland,5-Apr-12,10-Apr-12
Brussels,1-Apr-12,3-Apr-12
Cleveland,24-Jan-12,26-Jan-12
Darjeeling,18-Jan-12,19-Jan-12
Erehwon,8-Feb-12,10-Feb-12
Florence,5-Mar-12,7-Mar-12
Gandalf,25-Mar-12,27-Mar-12")

trips <- read.csv(con, header=TRUE)
trips$From <- as.Date( dmy(trips$From) )
trips$To <- as.Date( dmy(trips$To) )

# draw the Time Series data
p <- ggplot(track, 
            aes(x = Date, 
                y = value, 
                colour=variable,
                group = variable))
p <- p + geom_smooth()
p <- p + geom_point(size=3)
p <- p + geom_vline(data=trips, 
                    aes(xintercept=From),
                    colour='steelblue') # departure dates
p <- p + geom_vline(data=trips, 
                    aes(xintercept=To),
                    colour='grey80') # return dates
yrng <- c(80,120)
p <- p + scale_y_continuous(limits=yrng)

p # this works up to here, using vlines (ugly, but makes the point)

# ----------- THIS geom_rect() STOPS THIS PLOT BEING RENDERED
p2 <- p + geom_rect(aes(NULL, NULL, xmin=From, xmax=To),
                   ymin=yrng[1], ymax=yrng[2],
                   data=trips)

p2 # this gives an error message
# Error in eval(expr, envir, enclos) : object 'variable' not found

# ----------- THIS NOW WORKS, THANKS TO @smu, SEE BELOW
p2 <- p + geom_rect(aes(NULL, NULL, xmin=From, xmax=To, 
                   colour=NULL, group=NULL),
                   ymin=yrng[1], ymax=yrng[2],
               fill='orange',
               alpha=0.4,
               data=trips)
p2 # and I have added fill and alpha to prettify it a bit

正如@tyler-rinker 所建议的，我编辑了代码以包含原始错误和@smu 指出的解决方案。谢谢。

Answer 1

在第一 ggplot调用你图colour到“变量”。此映射仍然存在于 geom_rect 中。调用，但“trips”中不存在名为“variable”的变量。我建议使用以下方法取消映射颜色:

p2 <- p + geom_rect(aes(NULL, NULL, xmin=From, xmax=To, colour=NULL),
               ymin=yrng[1], ymax=yrng[2],
               data=trips)

(还没有尝试过，但我想它应该可以工作)。