在R/ggplot2中绘制多个图并保存结果

我正在创建一个(gg)绘图对象列表,然后尝试在单个图中查看所有绘图.从问题:如何使用grid.arrange安排任意数量的ggplots？,以及如何使用grid.arrange排列图表的变量列表？,我希望下面的代码可以解决这个问题(跳过最后几行代码实际执行多绘图的事情).

#!/usr/bin/Rscript
library(ggplot2)
library(reshape)
library(gridExtra)
args <- commandArgs(TRUE);
# Very first argument is the directory containing modelling results
setwd(args[1])
df = read.table('model_results.txt', header = TRUE)
df_actual = read.table('measured_results.txt', header = TRUE)
dfMerge = merge(df, df_actual, by = c("Clients", "MsgSize", "Connections"))

# All graphs should be stored in the directory pointed by second argument
setwd(args[2])

# Plot the results obtained from model for msgSize = 1, 1999
# and connections = 10, 15, 20, 25, 30, 50

msgSizes = c(1, 1999)
connVals = c(5, 10, 15, 20, 25, 30, 50)

cmp_df = data.frame(dfMerge$Clients, dfMerge$MsgSize, dfMerge$Connections, dfMerge$PThroughput, dfMerge$MThroughput)
colnames(cmp_df) = c("Clients", "MsgSize", "Connections", "ModelThroughput", "MeasuredThroughput")
cmp_df_melt = melt(cmp_df, id = c("Clients", "MsgSize", "Connections"))
colnames(cmp_df_melt) = c("Clients", "MsgSize", "Connections", "Variable", "Value")

plotList = list()
for (i in msgSizes) {
    msg_Subset = subset(cmp_df_melt, MsgSize == i)

    for (j in connVals) {
        plotData = subset(msg_Subset, Connections == j)

        filename = paste("Modelling.",i, ".", j, ".png", sep = '') 
        list_item = ggplot(data = plotData, aes(Clients, y = Value, color = Variable)) +
            geom_point() +
            geom_line() + 
            xlab("Number of Clients") +
            ylab("Throughput (in ops/second)") +
            labs(title = paste("Message Size = ", i, ", Connections = ", j), color = "Legend")

        plotList = c(plotList, list_item)
        # ggsave(file = filename)

    }
}

# Plot all graphs together
pdf("Summary.pdf")
list_len = length(plotList)
nCol = floor(sqrt(list_len))
do.call(grid.arrange, c(plotList, list(ncol = nCol)))
dev.off() 

相反,我遇到以下错误:

Error in arrangeGrob(..., as.table = as.table, clip = clip, main = main,  : 
  input must be grobs!
Calls: do.call ->  -> grid.draw -> arrangeGrob
Execution halted

我做错了什么,特别是因为两个相关的问题表明了同样的事情？此外,应该进行哪些更改才能将图形实际保存在文件中？