chem.R

library(ggplot2)
library(gridExtra)
library(grid)
library(tikzDevice)


chem <- "\\setatomsep{1em}\\chemfig{*6(-=-=(-[:90]N-[:30](=[:90]O)-[:-30](=[:-90]O)-[:30]N-[:-30]-[:30]-[:-30]N-[:30](=[:90]O)-[:-30](=[:-90]O)-[:30]N-[:-30](-[:-90]-[:-30](-[:30])-[:-90])-[:30](=[:90]O)-[:-30]O-[:30])-*6(-(=O)-*6(-=-=-=)--(=O)-)=)}"

p <- ggplot() + xlab('some chemistry') + ylab("some value") + 
  theme(plot.background = element_rect(colour = 'black', fill=NA),
        plot.tag = element_text(margin=margin(0,0,0,0)),
        plot.tag.position = c(0,1))

tikz(file = 'test.tex', standAlone = TRUE, width = 6, height=4,
     engine = 'xetex', bareBones = FALSE, 
     packages = c("\\usepackage{chemfig}",
                  "\\usepackage{preview}",
                  "\\usepackage[default,osf]{sourcesanspro}"))

grid.arrange(grobTree(rectGrob(),
                      textGrob("\\textbf{a}", vjust=1,hjust=0,x=0,y=1,
                               gp=gpar(fontsize=14)),
                      textGrob(chem)), 
             p + labs(tag = "\\textbf{b}"), 
             p + labs(tag = "\\textbf{c}"), 
             layout_matrix = cbind(c(1,2),c(3,3)),
             heights = c(2,5))

dev.off()