Why are they getting joined? The values are different

X <- c('1.07', '2.14', '3.22', '4.29')
Y <- c(0.298, 0.272, 0.271, 0.270)
Y1 <- c(0.274, 0.269, 0.255, 0.249)

par(mar = c(0.1, 0.1, 0.1, 0.1) + 2)   

plot(X, Y, xlab= "Height of the VG (in mm)", ylab= "Drag Co-efficient (Cd)", 
      main = "Height of the VG vs. Drag Co-efficient", type= "o", pch = 20, 
      lwd = 2, col = "red")

par(new = TRUE) 

plot(X, Y1, xlab= "", ylab= "", main = "", axes = FALSE, type= "o", 
    pch = 20, lwd = 2, col = "blue")

I'm trying to generate two y axes. But, both the lines are getting joined, even though with different value.

The problem is in the y axis limits, the plots are not using the same values. Below I first create a vector ylim with values common to Y and Y1, then use it in the plots.

X <- c('1.07', '2.14', '3.22', '4.29')
Y <- c(0.298,0.272,0.271,0.270)
Y1 <- c(0.274,0.269,0.255,0.249)

# save the default values
old_par <- par(mar = c(0.1, 0.1, 0.1, 0.1) + 2)   

# define the y axis limits
ylim <- range(c(Y, Y1))

plot(
  X, Y, 
  # set the y axis limits
  ylim = ylim,
  xlab = "Height of the VG (in mm)", 
  ylab= "Drag Co-efficient (Cd)", 
  main = "Height of the VG vs. Drag Co-efficient",
  type= "o",
  pch = 20, 
  lwd = 2, 
  col = "red"
)

par(new = TRUE) 

plot(
  X, Y1,
  # set the y axis limits to the
  # same value as above
  ylim = ylim,
  xlab= "", 
  ylab= "", 
  main = "", 
  axes = FALSE, 
  type = "o", pch = 20, 
  lwd = 2, 
  col = "blue"
)

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# restore default values
par(old_par)

^{Created on 2023-06-12 with reprex v2.0.2}