double caption in finalize

[matthewstern]

A common use of the caption grob produced by finalize_plot is to provide both a "note" and a "source" that should be a bit separate from each other. Most examples in the cookbook, for example, include <br><br> between text to create the necessary visual spacing.

A logical and easy add would be to allow the caption argument to take a vector of text entries, which finalize_plot would then combine with the necessary line breaks in the backend.

[nmpeterson]

As long as it remains backwards-compatible (i.e. a single string can still be supplied to the parameter, not wrapped in a c()) I have no problem with this. I guess it would just be a matter of checking whether the supplied value is a vector and, if so, returning cat(the_vector, sep = "<br><br>")?

[matthewstern]

yeah, I'm not even sure you'd need to check:

> caption <- c("This is a note.", "This is a source.")
> paste(caption, collapse = "<br><br>")
[1] "This is a note.<br><br>This is a source."

> caption <- "This is a note."
> paste(caption, collapse = "<br><br>")
[1] "This is a note."