Some older versions of gnuplot used rand(x>0) to produce sequential pseudo-random numbers. The current behavior is as follows:
`rand(0)` returns a pseudo random number in the interval [0:1] generated
from the current value of two internal 32-bit seeds.
`rand(-1)` resets both seeds to a standard value.
`rand(x)` for x>0 sets both seeds to a value based on the value of x.
`rand({x,y})` for x>0 sets seed1 to x and seed2 to y.