This function adds a plot label to the upper left corner of a graph (or an arbitrarily specified position). It takes all the same parameters
as draw_text, but has defaults that make it convenient to label graphs with letters A, B, C, etc. Just like draw_text(),
it can handle vectors of labels with associated coordinates.
Usage
draw_plot_label(label, x = 0, y = 1, hjust = -0.5, vjust = 1.5,
size = 16, fontface = "bold", ...)
Arguments
label
String (or vector of strings) to be drawn as the label.
x
The x position (or vector thereof) of the label(s).
y
The y position (or vector thereof) of the label(s).