[R] jitter-bug? problematic behaviour of the jitter function

[Duncan Murdoch]

On 23/09/2020 6:32 a.m., Martin Keller-Ressel wrote:
> Dear all,
> 
> i have noticed some strange behaviour in the „jitter“ function in R.
> On the help page for jitter it is stated that
> 
> "The result, say r, is r <- x + runif(n, -a, a) where n <- length(x) and a is the amount argument (if specified).“
> 
> and
> 
> "If amount is NULL (default), we set a <- factor * d/5 where d is the smallest difference between adjacent unique (apart from fuzz) x values.“
> 
> This works fine as long as there is no (very) large outlier
> 
>> jitter(c(1,2,10^4))  # desired behaviour
> [1]    1.083243    1.851571 9999.942716
> 
> But for very large outliers the added noise suddenly ‚jumps‘ to a much larger scale:
> 
>> jitter(c(1,2,10^5)) # bad behaviour
> [1] -19535.649   9578.702 115693.854
> # Noise should be of order (2-1)/5  = 0.2 but is of much larger order.
> 
> This probably does not matter much when jitter is used for plotting, but it can cause problems when jitter is used to break ties.

I think this is kind of documented:  "apart from fuzz" is what counts. 
If you look at the code for jitter, you'll see this important line:

  d <- diff(xx <- unique(sort.int(round(x, 3 - floor(log10(z))))))

By the time you get here, z is the length of the rante of the data, so 
it's 99999 in your example.  The rounding changes your values to 
0,0,1e5, so the smallest difference is 1e5.

Duncan Murdoch