yticks = c('A', 'B', 'C')
x = c(0, 1, 2, 3, 4, 5)
y = c(1, 2, 3, 2, 1, 3)
plot(y = y, x = x, yaxt = 'n', type='b') # yaxt='n' removes the y axis values
axis(side = 2, at = c(1,2,3), labels = yticks)