Labels outside of plots - r

Question

I am hoping to add the labels "a)" and "b)" to my two plots so that I can differentiate and discuss them more effectively when writing up. Ive tried to do this through the text and legend functions but I'm not getting any good results. Ideally I would have the a) in the very top left of the ep.var.hist plot (1st plot) and the b) in the very top left of the tp.var.hist plot (2nd plot), with the labels sitting outside the actual plot and above the y-axis labels.
My code is below
par(mfrow=c(2,1), mar=c(4,4,0.9,4))
ep.var.hist<-hist(data.ep, breaks=5, xlim=c(0,0.011), ylim=c(0,6000), xlab=NULL, main=NULL)
tp.var.hist<-hist(data.tp, breaks=66, xlim=c(0,0.011), ylim=c(0,6000), xlab="Variance", main=NULL)

Answer 1

Working with the cowplot package, designed to ease the process for producing publication ready plots.
library(cowplot)
library(ggplot2)
sepal <- ggplot(data = iris, aes(x = Species, y = Sepal.Length)) +
geom_bar(stat = "identity") +
theme(text = element_text(margin = margin(), debug = FALSE))
petal <- ggplot(data = iris, aes(x = Species, y = Petal.Length)) +
geom_bar(stat = "identity") +
theme(text = element_text(margin = margin(), debug = FALSE))
plot_grid(sepal, petal, labels = c("A", "B"))
plot_grid and save_plot (a polished version of ggsave) are my two favorite cowplot functions. I highly recommend looking at the help pages for more options and customization.
If you really want to keep it to just graphics try this solution, I think you are looking for adj = 0:
par(mfrow=c(2,1), mar=c(4,4,0.9,4))
petal <- hist(iris$Petal.Length, main = "Petal", adj = 0)
sepal <- hist(iris$Sepal.Length, main = "Sepal", adj = 0)
Full disclaimer I would strongly consider using ggplot2 in the long term like #rosscova suggested. You will have a lot more options for controlling the details of your plots, plus lots of modern visualizations that base R just can't do. There is a reason why ggplot2 is so popular :)

Answer 2

EDIT: I apologise, I went ahead and answered without realising you said "outside of plots", which I don't think my answer can achieve.
I don't know how to do this in base, but ggplot2 has the annotate function to achieve what you're after. Here's an example from which you can start playing (I've added a few bits you might want to help get you started):
library( ggplot2 )
plot <- ggplot( diamonds ) +
geom_histogram( aes( carat ), bins = 30 ) +
annotate( "text", label = "label here", x = 1, y = 7500, col = "red" ) +
annotate( "text", label = "and another", x = 2, y = 5500, col = "blue" )
plot <- plot +
xlim( 0, 3 ) +
ggtitle( "Main title" ) +
xlab( "label x" ) +
ylab( "label y" )
plot
Adjust the x and y values within the annotate function to move the label around. You can add as many of these as you like by adding more calls to annotate.