R horizontal barplot with aligned plot ontop - r

Question

I am having trouble getting the spacing right on a plot on top of a horizontal barplot. It is the same general issue as described here:
http://www.r-bloggers.com/adding-lines-or-points-to-an-existing-barplot/
But I am trying to use "plot" instead of "points" or "lines". Is there a trick for using plot to get the spacing of the bars and the points to match?
Code:
barplot(df$DIC_mM,col=scalegreen, xlab="DIC mM", horiz=TRUE, xlim=c(0,0.7),
col.axis="white", col.lab="white", axes=FALSE, border="white")
axis(1,line=1,col="white",col.ticks="white",col.axis="white")
par(new = TRUE)
plot(df$d13DIC,df$Order, type="p", axes = FALSE, bty = "n", xlab ="",
col="deepskyblue2", lwd=5, xlim=c(-50,170), lend=2, col.lab="white", ylab="")
axis(3,at = c(-50,0,50,100,150), line=1, col="deepskyblue2", col.ticks="deepskyblue2",
col.axis="deepskyblue2")
mtext(expression(paste(delta ^{13},'DIC'," \u0028","\u2030","\u0029")), 3,
line=-0.5,at=50,col="deepskyblue2", cex=0.75)

Answer 1

Is there a reason why you don't want to use points to add the points? If you're willing to use points you can do it like this:
Create barplot and save the y-coordinates of the bars to y. You haven't provided sample data, so I'll use the built-in mtcars data frame:
y = barplot(mtcars$mpg[1:10], horiz=TRUE)
Now add the points. We use y as the y values, because those are the coordinates of the midpoints of each bar:
points(sqrt(mtcars$mpg[11:20]), y, col="red", pch=16, cex=2)
When you use par(new=TRUE) and then call plot again, you're overlaying a new plot with a new coordinate system that in general will be different from the original coordinate system.

Answer 2

This is what worked, based on this post suggested by eipi10: midpoints returned by barplot function do not actually line up with midpoints of bars
mp<-barplot(df$DIC_mM,col=scalegreen, xlab="DIC mM", horiz=TRUE, xlim=c(0,0.7), col.axis="white", col.lab="white", axes=FALSE, border="white", ylim=c(0,length(df$DIC_mM)+2))
axis(1,line=1,col="white",col.ticks="white",col.axis="white")
par(new = TRUE)
plot(df$d13DIC, mp, type="p", axes = FALSE, bty = "n", xlab ="",col="deepskyblue2", lwd=5, xlim=c(-50,170), lend=2, col.lab="white", ylab="", ylim=c(0,length(df$DIC_mM)+2))
axis(3,at = c(-50,0,50,100,150),line=1,col="deepskyblue2",col.ticks="deepskyblue2",col.axis="deepskyblue2")
mtext(expression(paste(delta ^{13},'DIC'," \u0028","\u2030","\u0029")),3,line=-0.5,at=50,col="deepskyblue2", cex=0.75)