Question

I’m trying to use a named character vector to hold a custom color palette, so I can say, e.g. palette[‘red’] instead of repeating “#dc322f” all over the place.

However, I don’t seem to be able to use an element of that vector as an argument to par() (though it can be used elsewhere).

Here’s an example. It’ll create a graph with green dots, but the par() call fails and the background is white. Note that I can set parameters using the palette vector from within the plot() call:

> palette <- c('#002b36','#dc322f','#859900')
> names(palette) <- c('black','red','green')
> par(bg=palette['red'])                                                                                                                                
Warning message:
In par(bg = palette["red"]) : "bg.red" is not a graphical parameter
> plot(1:10,1:10,col=palette['green'])
> # (White graph with green dots appears) 

When I use a named numeric vector, however, it works:

> palette <- 1:3                                                                                                                                          
> names(palette) <- c('black','red','green')                                                                                                              
> par(bg=palette['red'])                                                                                                                             
> # (no error here -- it worked.)
> plot(1:10,1:10,col=palette['green'])
> # (Red graph with green dots appears)

I am fairly new to R, and it seems that I might be missing something fundamental. Any idea what’s happening here?

Answer 1

Use unname, so that the item passed to par is just the character vector defining the colour, not a named element

palette <- c('#002b36','#dc322f','#859900')
names(palette) <- c('black','red','green')
par(bg=unname(palette['red'])) 
plot(1:10,1:10,col=palette['green'])

As to why?

within par, if all the arguments are character vectors then

if (all(unlist(lapply(args, is.character)))) 
            args <- as.list(unlist(args))

The way as.list(unlist(args)) works if args is a named character vector is

args <- list(bg = palette['red'])
as.list(unlist(args))
$bg.red
[1] "#dc322f"

and bg.red is not a valid par.

if the line were something like

setNames(as.list(unlist(args, use.names = F)), names(args))

then it might work in some cases (although not if any of the named elements of arg had length >1)