Aligning or scaling two y axes: Zero values of y1 & y2 should be exactly opposite in R

Load sample data and plot:

x <- 1:5
y1 <- rnorm(5)
y2 <- rnorm(5,20)
par(mar=c(5,4,4,5)+.1)
plot(x,y1,type="l",col="red")
par(new=TRUE)
plot(x, y2,,type="l",col="blue",xaxt="n",yaxt="n",xlab="",ylab="")
axis(4)
mtext("y2",side=4,line=3)
legend("topleft",col=c("red","blue"),lty=1,legend=c("y1","y2"))`

Above code will you this picture

What am I stuck at?

I want to align the two y-axes such that zero values on y1 and y2 to be opposite to each-other.

Any help?

Please let me know if you have any questions or clarification. Thanks!

Solution

Below listed code should work for aligning two-y-axes to baseline; however, y1 has strange values rather than 0 (in some plots #remember I am plotting 80 two-y-axes plots), this will require adjustment in my algorithm for setting y-axes range, but otherwise plot(s) seem ok. Let me know if you find out the reason or have any clarification(s).

rangeY1=range(df$y1, na.rm=TRUE, finite=TRUE)

rangeY2=range(df$y2, na.rm=TRUE, finite=TRUE)

# Scales of the given ranges
sumY1= rangeY1[2]-rangeY1[1]
sumY2= rangeY2[2]-rangeY2[1]
# Relative part of the positive and negative axis, only temporary variables
R=abs(rangeY1[1]/sumY1)
S=abs(rangeY2[1]/sumY2)
K=abs(rangeY1[2]/sumY1)
L=abs(rangeY2[2]/sumY2)
#Select new minimum values for the ranges
if(R>S){
  new_miny1=rangeY1[1]
  new_miny2=sumY2*rangeY1[1]/sumY1 }
if(S>R){
  new_miny1=sumY1*rangeY2[1]/sumY2
  new_miny2=rangeY2[1] }
if(S==R){
  new_miny1=rangeY1[1]
  new_miny2=rangeY2[1] }
#Select new maximum values for the ranges
if(K>L){
  new_maxy1=rangeY1[2]
  new_maxy2=sumY2*rangeY1[2]/sumY1 }
if(L>K){
  new_maxy1=sumY1*rangeY2[2]/sumY2
  new_maxy2=rangeY2[2] }
if(K==L){
  new_maxy1=rangeY1[2]
  new_maxy2=rangeY2[2] }
#"New ranges"
new_rangeY1=c(new_miny1, new_maxy1)
new_rangeY2=c(new_miny2, new_maxy2)