"for"循环只添加最终的ggplot图层

Question

简介:当我使用"for"循环将图层添加到小提琴图(在ggplot中)时,添加的唯一图层是由最终循环迭代创建的图层.然而,在模拟循环将生成的代码的显式代码中,添加了所有层.

详细信息:我正在尝试创建具有重叠层的小提琴图,以显示估计分布对于多个调查问题响应重叠或不重叠的程度,按地点分层.我希望能够包含任意数量的地方,因此每个地方我都有一列数据框,并且我尝试使用"for"循环来为每个地方生成一个ggplot图层.但是循环只会从循环的最后一次迭代中添加该层.

此代码说明了问题,以及一些失败的建议方法:

library(ggplot2) 

# Create a dataframe with 500 random normal values for responses to 3 survey questions from two cities
topic <- c("Poverty %","Mean Age","% Smokers")
place <- c("Chicago","Miami")
n <- 500
mean <- c(35,  40,58,  50, 25,20)
var  <- c( 7, 1.5, 3, .25, .5, 1)
df <- data.frame( topic=rep(topic,rep(n,length(topic)))
                 ,c(rnorm(n,mean[1],var[1]),rnorm(n,mean[3],var[3]),rnorm(n,mean[5],var[5]))
                 ,c(rnorm(n,mean[2],var[2]),rnorm(n,mean[4],var[4]),rnorm(n,mean[6],var[6]))
                )
names(df)[2:dim(df)[2]] <- place  # Name those last two columns with the corresponding place name.
head(df) 

# This "for" loop seems to only execute the final loop (i.e., where p=3)
g <- ggplot(df, aes(factor(topic), df[,2]))
for (p in 2:dim(df)[2]) {
  g <- g + geom_violin(aes(y = df[,p], colour = place[p-1]), alpha = 0.3)
}
g

# But mimicing what the for loop does in explicit code works fine, resulting in both "place"s being displayed in the graph.
g <- ggplot(df, aes(factor(topic), df[,2]))
g <-   g + geom_violin(aes(y = df[,2], colour = place[2-1]), alpha = 0.3)
g <-   g + geom_violin(aes(y = df[,3], colour = place[3-1]), alpha = 0.3)
g

## per http://stackoverflow.com/questions/18444620/set-layers-in-ggplot2-via-loop , I tried 
g <- ggplot(df, aes(factor(topic), df[,2]))
for (p in 2:dim(df)[2]) {
  df1 <- df[,c(1,p)]
  g <- g + geom_violin(aes(y = df1[,2], colour = place[p-1]), alpha = 0.3)
}
g
# but got the same undesired result

# per http://stackoverflow.com/questions/15987367/how-to-add-layers-in-ggplot-using-a-for-loop , I tried
g <- ggplot(df, aes(factor(topic), df[,2]))
for (p in names(df)[-1]) {
  cat(p,"\n")
  g <- g + geom_violin(aes_string(y = p, colour = p), alpha = 0.3)  # produced this error: Error in unit(tic_pos.c, "mm") : 'x' and 'units' must have length > 0
  # g <- g + geom_violin(aes_string(y = p            ), alpha = 0.3)  # produced this error: Error: stat_ydensity requires the following missing aesthetics: y
}
g
# but that failed to produce any graphic, per the errors noted in the "for" loop above

Answer 1

发生这种情况的原因是由于ggplot"懒惰评估".当ggplot以这种方式使用时,这是一个常见的问题(将这些层分别放在一个循环中,而不是ggplot像你在@ hrbrmstr的解决方案中那样使用它).

ggplot将参数存储aes(...)为表达式,并仅在渲染绘图时对其进行求值.所以,在你的循环中,类似于

aes(y = df[,p], colour = place[p-1])

按原样存储,并在循环完成后渲染绘图时进行评估.此时,p = 3,因此所有图都以p = 3呈现.

因此,执行此操作的"正确"方法是melt(...)在reshape2包中使用,以便将数据从宽格式转换为长格式,并让您ggplot管理图层.我把"正确"放在引号中,因为在这种特殊情况下有一个微妙之处.在使用融合数据框计算小提琴的分布时,ggplot使用总计(芝加哥和迈阿密)作为比例.如果你想要基于单独缩放频率的小提琴,你需要使用循环(遗憾地).

延迟评估问题的方法是在data=...定义中对循环索引进行任何引用.这不是作为表达式存储的,实际数据存储在绘图定义中.所以你可以这样做:

g <- ggplot(df,aes(x=topic))
for (p in 2:length(df)) {
  gg.data <- data.frame(topic=df$topic,value=df[,p],city=names(df)[p])
  g <- g + geom_violin(data=gg.data,aes(y=value, color=city))
}
g

这会产生与你相同的结果.请注意,索引p不会显示在aes(...).

更新:关于scale="width"(在评论中提到)的说明.这导致所有小提琴具有相同的宽度(见下文),这与OP的原始代码中的缩放不同.IMO这不是一个可视化数据的好方法,因为它表明芝加哥集团有更多的数据.

ggplot(gg) +geom_violin(aes(x=topic,y=value,color=variable),
                        alpha=0.3,position="identity",scale="width")