[R] plot representation of calculated value known to be 7.4

[Troels Ring]

Dear friends - merry Christmas and thanks a lot for much help during the 
year!

In the example below I fail to understand how the calculated value pH is 
represented in a simple plot - also included. The calculations are 
useful in practice and likely to be right in principle but I cannot see 
how this occurs: why a calculated value of 7.4 known as numeric is not 
simply plotted as such. It happened on Windows both 7 and 10 with R 
version 3.4.1.

All best wishes

Troels

ff <- function(H,SID,ATOT,ka)  H + SID - kw/H - ka*ATOT/(H+ka)
  ka <- 1e-7
  kw <- 1e-14
  ATOT <- seq(0,0.3,length=100)*1e-3

  for (i in 1:length(ATOT))  {
  SID[i] <- uniroot(ff,c(-1,1),tol=.Machine$double.eps,maxiter=100000,ka=ka,
ATOT=ATOT[i], H = 10^-7.4)$root}
ATOT

#confirm pH 0 7.4

H <- c()
  for (i in 1:length(ATOT))  {
  H[i] <- 
uniroot(ff,c(1e-19,1),tol=.Machine$double.eps,maxiter=100000,ka=ka,
ATOT=ATOT[i], SID = SID[i])$root}

(pH <- -log10(H))
plot(pH)
str(pH)
# num [1:100] 7.4 7.4 7.4 7.4 7.4 ...
z <- rep(7.4,length(ATOT))
all.equal(z,pH)
#TRUE
points(z,col="red")