ggplot figure number in for loop in R

Question

I have got the following data:

    Detector=c("A", "A", "A", "A","B", "B", "B", "B","C", "C", "C", "C")
    hours=rep(c(0,4,24,48),3)
    LoD=c(rep(25,4),rep(40,4),rep(34,4))
    Ct=c(24,24.2,24.3,24,43,42,43,41,35,37,35,35.6)
    Sample.Type=c(rep("T",2),rep("P",4),rep("T",5), rep("P",1))
    d4.1=data.frame(Detector,hours, Ct, LoD, Sample.Type)

I want to plot Ct vs hours as a simple scatterplot, as a loop for each Detector, and show each detector's lod value as a horizontal line on the plot. I use the following code which works perfectly well:

    for(i in unique(d4.1$Detector)) {
    print(ggplot(d4.1[d4.1$Detector==i], aes(d4.1$hours[d4.1$Detector==i], 
    d4.1$Ct[d4.1$Detector==i], colour = d4.1$Sample.Type[d4.1$Detector==i]))+
    geom_point()+
    geom_hline(yintercept=mean(d4.1$LoD[d4.1$Detector==i]))+
    ggtitle(paste("Figure", j,i, sep=" "))+
    theme(plot.title = element_text(hjust = 
    0.5),plot.caption=element_text(hjust = 0))+
    labs(x="Hours/Time Points", y="Ct",caption=paste("The solid black line 
    is the LoD(=",mean(d4.1$LoD[d4.1$Detector==i]),") for",i,sep=" "), 
    colour="Sample Types"))}

What I need help with is the ggplot's titles. Each of the plots generated is labeled with A,B and C but i'd like to add the following text to it:

    Figure 1: A
    Figure 2: B
    Figure 3: C

I tried adding a "j" sequence of 1-3 and paste it in title (below) which didnt work. It gave me 9 plots (Figure 1 A,Figure 1 B, Figure 1 C, Figure 2 A...etc)

     for(i in unique(d4.1$Detector)){for (j in seq(1:3)){print(ggplot(d4.1[which(d4.1$Detector==i),], aes(hours,Ct, colour = Sample.Type))+geom_point()+geom_hline(yintercept=mean(LoD))+ggtitle(paste("Figure",j,i,sep=" "))+theme(plot.title = element_text(hjust = 0.5),plot.caption=element_text(hjust = 0))+labs(x="Hours/Time Points", y="Ct",caption=paste("The solid black line is the LoD(=",mean(LoD),") for",i,sep=" "), colour="Sample Types"))}}

How do I add a sequence of numbers (1,2,3...) to ggplot titles in a loop? Thank you very much.

Answer 1

Your code was not reproducible because the way you programmed your loop. You subset the dataframe first and then within your aes() ask for subsets that no longer exists.

Although I find Kristoffer's and Adam's answers more elegant than mine here is a crude version that uses your original loop. I use the fact that factor level A corresponds to the first level, by converting the factor to a numeric.

for(i in unique(d4.1$Detector)) {


 print(ggplot(d4.1[d4.1$Detector==i,], aes(hours, 
                                           Ct, colour = Sample.Type))+
          geom_point()+
          geom_hline(yintercept=mean(d4.1$LoD[d4.1$Detector==i]))+
          ggtitle(paste("Figure", as.numeric(d4.1$Detector[d4.1$Detector==i]), ":" , i, sep=" "))+
            theme(plot.title = element_text(hjust = 
                                            0.5),plot.caption=element_text(hjust = 0))+
          labs(x="Hours/Time Points", y="Ct",caption=paste("The solid black line 
                                                           is the LoD(=",mean(d4.1$LoD[d4.1$Detector==i]),") for",i,sep=" "), 
               colour="Sample Types"))}

Answer 2

If you store unique Detector in a variable (detectors) and use seq_along you will get indexes 1 to 3 (x in the function). You can then create your three plots like this:

detectors <- unique(d4.1$Detector)
lapply(seq_along(detectors), function (x) {
  selected_detector <- detectors[[x]]
  df_plot <- d4.1[d4.1$Detector == selected_detector, ]
  p <- ggplot(df_plot, aes(x = hours, y = Ct, colour = Sample.Type)) +
      geom_point() + geom_hline(yintercept = mean(df_plot$LoD)) +
      ggtitle(paste("Figure", x, selected_detector, sep = " ")) +
      theme(plot.title = element_text(hjust = 0.5),
        plot.caption = element_text(hjust = 0))+
        labs(x = "Hours/Time Points", y = "Ct",
          caption=paste("The solid black line is the LoD(=",
            mean(df_plot$LoD),") for", selected_detector, sep=" "),
          colour="Sample Types")
  print(p)
})

Answer 3

Since your code is not reproducible, I'd decided to give an alternate example using the iris data.

(i) split iris into a list of 3 subset, based on Species. This is similar to d4.1[d4.1$Detector==i,] in the code provided

df <- split(iris, iris$Species)

(ii) write a generic plot function that takes in different subset

plotFn <- function(df){
              ggplot(df, aes(Sepal.Length, Petal.Length)) + 
              geom_point() + 
              geom_hline(yintercept = mean(df[,"Petal.Length"]))
          }

(iii) get the list of ggplot object and then do a lapply to individually add in the Figure x: y format.

p <- lapply(df, plotFn)

lapply(1:length(p), function(x){
          p[[x]] + 
          ggtitle(paste0("Figure ", x, ": ", names(p)[x]))
       })

This will provide three plots: