Properly labeling abline with text?

The result of the code shown below is posted at the following URL:
http://n2.nabble.com/Trying-to-properly-label-abline-td2524629.html

I would like to figure out a better way to label the horizontal  
abline.

I tried multiplying the text y position by a scale, but this didn't  
always put the text in a useful position.   I guess I am curious if  
there is a preferred procedure for labeling the abline.  Is there a  
way to "fill" the text label and also put a box around the text  
label?  I guess I would also like to have the box around the text  
match the abline format.

Thank you for any info or insights.


x_vals<-1:1000
y_vals<-rnorm(1000)

plot(x_vals, y_vals)

top_label<-paste("Top Label = ", format(max(y_vals), digits=2),  
sep="")
abline(h=max(y_vals), col="darkred")
text(max(x_vals), max(y_vals), top_label, pos=2)

bottom_label<-paste("Bottom Label = ", format(min(y_vals),digits=2),  
sep="")
abline(h=min(y_vals), col="darkred")
text(max(x_vals), min(y_vals), bottom_label, pos=2)