First, save your barplot as some object. So you will get coordinates of the middle points. Then to add line you can use also function lines()
and just multiply Change
values with 1000.
Then for axis()
function supply at=
values and labels=
the same as at=
, just divided by 1000.
x<-barplot(Sales,ylim=c(0,1000))
lines(x,Change*1000,type="b")
axis(4,at=seq(0,800,200),labels=seq(0,800,200)/1000)