Figure caption scientific names + symbols in textGrob gtable

Question

First of all I would like to thank Sir Baptiste for helping me improve my R script by adding a caption at the bottom left the of the combined plots using gtable/textGrob as

Answer 1

I think a good solution would rely on LaTeX or similar for the text rendering and particularly the tricky issue of line-wrapping, but something could be designed at R level to facilitate the inclusion of plotting symbols that correspond to a given graphic. Something along those lines,

gl = extract_legend_grobs(p)
caption = caption_plot("Figure 1. We are referring to the points {{gl$points[supp == OG'']}}. 
 The theoretical model is shown as {{gl$lines[type == 'theory']}}.", gl)

print(caption, output="latex")
## "Figure 1. We are referring to the points \includegraphics{gl_p_1.png}. 
## The theoretical model is shown as \includegraphics{gl_l_1.png}."

Interesting thought, but probably a lot of work to get it right.

A quick-and-dirty R graphics output could also be devised, though it's uncommon to want captions to be part of the figure (and R graphics isn't particularly good with text).

Here's a weak attempt at making a caption grob mixing symbols and text. Ideally the text would be split into individual words first (to offer more options for line breaks), but plotmath expressions make it inconvenient.

Next step would be to add a few convenient wrappers to generate common symbols, and to interleave the two lists of grobs.

library(grid)
library(gridExtra)

inwidth <- function(x, margin=unit(1,"mm")) {
  if(inherits(x, "text"))
    convertWidth(grobWidth(x)+margin, "in", valueOnly = TRUE) else
      convertWidth(unit(1,"line")+margin, "in", valueOnly = TRUE)
}

captionGrob <- function(..., width = unit(4, "in"), debug = FALSE){

  maxw <- convertWidth(width, "in", valueOnly = TRUE)
  lg <- list(...)
  lw <- lapply(lg, inwidth)
  stopifnot(all(lw < maxw))

  # find breaks
  cw <- cumsum(lw)
  bks <- which(c(0, diff(cw %% maxw))  < 0 )
  # list of lines
  tg <- list()
  starts <- c(1, bks)
  ends <- c(bks -1, length(lg))

  for(line in seq_along(starts)){
    ids <- seq(starts[line], ends[line])
    sumw <- do.call(sum,lw[ids])
    neww <- maxw - sumw # missing width to fill
    filler <- rectGrob(gp=gpar(col=NA, fill=NA), 
                             width=unit(neww, "in"), 
                             height=unit(1, "line"))
    grobs <- c(lg[ids], list(filler))

    # store current line
    tg[[line]] <- arrangeGrob(grobs=grobs, nrow = 1, 
                              widths = unit(c(lw[ids], neww), "in"))

  }

  # arrange all lines in one column
  grid.arrange(grobs=tg, ncol=1,
               heights = unit(rep(1, length(tg)), "line"))

  if(debug)  grid.rect(width=width, gp=gpar(fill=NA, lty=2))
}

tg <- lapply(c(expression(bold(Figure~1.)~italic(Those)~points), 
               "are important, ", "nonetheless", "and", "have value too."), 
             textGrob)
pGrob <- function(fill, size=1, ...){
  rectGrob(..., width=unit(size,"line"), height=unit(size,"line"), gp=gpar(fill=fill))
}
pg <- mapply(pGrob, fill=1:5, size=0.5, SIMPLIFY = FALSE)
grid.newpage()
captionGrob(tg[[1]], pg[[1]], pg[[2]], pg[[3]], tg[[2]], tg[[3]], pg[[4]], tg[[4]], pg[[5]], tg[[5]])

Answer 2

Based on the comments, I suggest the following strategy: create a dummy plot with your figure caption (text) as legend title, extract its legend, and place it at the bottom of your gtable.

library(grid)
library(gridExtra)
library(ggplot2)
library(gtable)

p1 <- ggplot()
p2 <- ggplot(ToothGrowth, aes(len, dose, shape=supp)) + geom_point() +
  theme(legend.position="bottom", 
        legend.background=element_rect(colour="black")) 

title <- expression("Figure 1. This "*italic(is)*" now a legendary caption")
dummy <- ggplotGrob(p2 + guides(shape = guide_legend(title = title)))

g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
caption <- gtable_filter(dummy,"guide")[["grobs"]][[1]]
caption$widths <- grid:::unit.list(caption$widths)
caption$widths <- unit.c(unit(0,"mm"), caption$widths[2], unit(1,"null"))

g <- rbind(g1, g2)
g <- gtable::gtable_add_rows(g, unit(2,"mm") + grobHeight(caption), -1)
g <- gtable::gtable_add_grob(g, caption, nrow(g), l = 4, r = ncol(g))
grid.newpage()
grid.draw(legend)
grid.draw(g)