Returns a vector of values that are pretty within each decade,
and that span the range of the input.
USAGE:
pretty.log(x, nint = 5)
REQUIRED ARGUMENTS:
x
vector of data; prettied values will cover the range of x.
Missing values (NAs) are allowed.
OPTIONAL ARGUMENTS:
nint
approximate number of intervals desired.
VALUE:
vector of approximately nint values that are ordered and
equally spaced within each decade over the range of the input x,
with the same spacing for each decade.
The individual values will differ by 1, 2 or 5 in each decade.
Negative, zero, and missing values will be dropped before calculation.