plot numeric vector with names as x ticks

You can use function axis() to add labels. Argument xaxt="n" inside plot() will make plot without x axis labels (numbers).

plot(quantity,xaxt="n")
axis(1,at=1:3,labels=names(quantity))

Do you look for barplot?

barplot(quantity)

And another option using lattice:

library(lattice)
barchart(quantity)