set plot panel size in ggplot tiff exports

Question

I am generating a set of plots within a for loop using ggplot2 and setting the overall ggplot export size with:

ppi <- 300
tiff(file_name, width=2*ppi, height=2*ppi, res=ppi)

However, the actual plot panel size changes when, for example, the number of digits shown along the y-axis changes.

How can this be modified to work for tiff exports?

Reproducible example here (edited to include set_panel_size function:

> CohData
     Name ScenID ScenName Year VarName Units value
1  Site01      0     Hind 2000     ANC ueq/L 28.23
2  Site02      0     Hind 2000     ANC ueq/L 35.34
3  Site03      0     Hind 2000     ANC ueq/L 31.73
4  Site04      0     Hind 2000     ANC ueq/L 54.21
5  Site05      0     Hind 2000     ANC ueq/L 28.23
6  Site01      1       BC 2020     ANC ueq/L 45.01
7  Site02      1       BC 2020     ANC ueq/L 32.86
8  Site03      1       BC 2020     ANC ueq/L 30.38
9  Site04      1       BC 2020     ANC ueq/L 53.01
10 Site05      1       BC 2020     ANC ueq/L 31.11
11 Site01      0     Hind 2000      pH        6.40
12 Site02      0     Hind 2000      pH        5.93
13 Site03      0     Hind 2000      pH        6.36
14 Site04      0     Hind 2000      pH        5.75
15 Site05      0     Hind 2000      pH        5.16
16 Site01      1       BC 2020      pH        6.63
17 Site02      1       BC 2020      pH        5.86
18 Site03      1       BC 2020      pH        6.33
19 Site04      1       BC 2020      pH        5.73
20 Site05      1       BC 2020      pH        5.23

   ## Iterate plots and write to tiff
theme_set(theme_bw(base_size = 8))    
ppi <- 300
CohData_nohind <- subset(CohData, ScenID > 0)
stream_vars <- list("ANC", "pH")
for (i in unique(CohData_nohind$ScenID)){
  subdata1 <- subset(CohData_nohind, ScenID == i)
  ScenName <- unique(subdata1$ScenName)
  subdata1 <- subset(CohData, ScenID == i | ScenID == 0)
  for (j in stream_vars){
    subdata2 <- subset(subdata1, VarName == j)
    units <- unique(subdata2$Units)
    for (k in unique(subdata2$Name)){
      subdata3 <- subset(subdata2, Name == k)

  if (j=='ANC'){
    file_name = paste("stream", k,i,j, ".tiff", sep="")
    #OLD Code tiff(file_name, width=2*ppi, height=2*ppi, res=ppi)
    print(ggplot(subdata3, aes(x = Year, y = value)) + geom_line() + expand_limits(y=c(0,100)) + 
            ggtitle(paste0(as.character(ScenName),":\n", as.character(k))) + ylab(paste0(j, " (", units, ")")))

    set_panel_size <- function(p=NULL, g=ggplotGrob(p), file=file_name, 
                               margin = unit(1,"mm"),
                               width=unit(4, "cm"), height=unit(4, "cm")){

      panels <- grep("panel", g$layout$name)
      panel_index_w<- unique(g$layout$l[panels])
      panel_index_h<- unique(g$layout$t[panels])
      nw <- length(panel_index_w)
      nh <- length(panel_index_h)

      # the following conversion is necessary
      # because there is no `[<-`.unit method
      # so promoting to unit.list allows standard list indexing
      g$widths <- grid:::unit.list(g$widths)
      g$heights <- grid:::unit.list(g$heights)

      g$widths[panel_index_w] <-  rep(list(width), nw)
      g$heights[panel_index_h] <-  rep(list(height), nh)
      class(g) <- c("fixed", class(g), "ggplot")
      if(!is.null(file))
        ggsave(file, g, 
               width = convertWidth(sum(g$widths) + margin, unitTo = "in", valueOnly = TRUE),
               height = convertHeight(sum(g$heights) + margin, unitTo = "in", valueOnly = TRUE))

      g
    }

    print.fixed <- function(x) grid.draw(x)
    #OLD Code: dev.off()

  } else if (j=='pH'){
    file_name = paste("stream", k,i,j, ".tiff", sep="")
    #OLD Code: tiff(file_name, width=2*ppi, height=2*ppi, res=ppi)
    print(ggplot(subdata3, aes(x = Year, y = value)) + geom_line() + expand_limits(y=c(3,7)) +
            ggtitle(paste0(as.character(ScenName),":\n", as.character(k))) + ylab(paste0(j, " (", units, ")")))

    set_panel_size <- function(p=NULL, g=ggplotGrob(p), file=file_name, 
                               margin = unit(1,"mm"),
                               width=unit(4, "cm"), height=unit(4, "cm")){

      panels <- grep("panel", g$layout$name)
      panel_index_w<- unique(g$layout$l[panels])
      panel_index_h<- unique(g$layout$t[panels])
      nw <- length(panel_index_w)
      nh <- length(panel_index_h)

      # the following conversion is necessary
      # because there is no `[<-`.unit method
      # so promoting to unit.list allows standard list indexing
      g$widths <- grid:::unit.list(g$widths)
      g$heights <- grid:::unit.list(g$heights)

      g$widths[panel_index_w] <-  rep(list(width), nw)
      g$heights[panel_index_h] <-  rep(list(height), nh)
      class(g) <- c("fixed", class(g), "ggplot")
      if(!is.null(file))
        ggsave(file, g, 
               width = convertWidth(sum(g$widths) + margin, unitTo = "in", valueOnly = TRUE),
               height = convertHeight(sum(g$heights) + margin, unitTo = "in", valueOnly = TRUE))

      g
    }

    print.fixed <- function(x) grid.draw(x)
    #OLD Code: dev.off()
   }
  }
 }
}