从x和y轴绘制带有其索引的通用标签

Question

I am new to R and I want to plot graph as example. My x and y values are as follow:

   a          b
orange     tomato
apple      butter 
tomato     graps
chiku      orange
graps      apple
potato     chiku
onion      butter
ginger     cheese
cheese     onion
butter     ginger

Now on the x-axis I want the the value (not the index but the names which are shown above as 'a') of a, and on the y-axis 'b' from above table.
Now I want to plot scatter points at intersecting point. For ex. in 'a' the index of 'orange' is 1 and in 'b' the index of 'orange' is 4, so in the graphIi want point at (1,4) the intersect point of orange.
Same for the rest of the values. I don't know how to plot this and I am suffering from past few days.

Answer 1

I changed one of the butters to potato since I assumed this was a typo.

dd <- read.table(header = TRUE, text = "a          b
orange     tomato
apple      butter 
tomato     graps
chiku      orange
graps      apple
potato     chiku
onion      potato
ginger     cheese
cheese     onion
butter     ginger")

dd <- within(dd, {
  x <- factor(a, levels = a)
  y <- factor(x, levels = b)
})

plot(idx <- sapply(dd[, c('x','y')], as.numeric))
text(idx, labels = dd$a, pos = 1, xpd = NA)

edit

plot(idx <- sapply(dd[, c('x','y')], as.numeric), xaxt = 'n', yaxt = 'n')
text(idx, labels = dd$a, pos = 1, xpd = NA)

axis(1, at = seq_along(dd$a), labels = dd$a)
axis(2, at = seq_along(dd$b), labels = dd$b, las = 1)