pretty {base}  R Documentation 
Compute a sequence of about n+1
equally spaced ‘round’
values which cover the range of the values in x
.
The values are chosen so that they are 1, 2 or 5 times a power of 10.
pretty(x, ...) ## Default S3 method: pretty(x, n = 5, min.n = n %/% 3, shrink.sml = 0.75, high.u.bias = 1.5, u5.bias = .5 + 1.5*high.u.bias, eps.correct = 0, ...)
x 
an object coercible to numeric by 
n 
integer giving the desired number of intervals. Noninteger values are rounded down. 
min.n 
nonnegative integer giving the minimal number of
intervals. If 
shrink.sml 
positive number, a factor (smaller than one)
by which a default scale is shrunk in the case when

high.u.bias 
nonnegative numeric, typically > 1.
The interval unit is determined as {1,2,5,10} times 
u5.bias 
nonnegative numeric
multiplier favoring factor 5 over 2. Default and ‘optimal’:

eps.correct 
integer code, one of {0,1,2}. If non0, an
epsilon correction is made at the boundaries such that
the result boundaries will be outside 
... 
further arguments for methods. 
pretty
ignores nonfinite values in x
.
Let d < max(x)  min(x)
≥ 0.
If d
is not (very close) to 0, we let c < d/n
,
otherwise more or less c < max(abs(range(x)))*shrink.sml / min.n
.
Then, the 10 base b
is
10^(floor(log10(c))) such
that b ≤ c < 10b.
Now determine the basic unit u as one of
{1,2,5,10} b, depending on
c/b in [1,10)
and the two ‘bias’ coefficients, h
=high.u.bias
and f =u5.bias
.
.........
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
axTicks
for the computation of pretty axis tick
locations in plots, particularly on the log scale.
pretty(1:15) # 0 2 4 6 8 10 12 14 16 pretty(1:15, high.u.bias = 2) # 0 5 10 15 pretty(1:15, n = 4) # 0 5 10 15 pretty(1:15 * 2) # 0 5 10 15 20 25 30 pretty(1:20) # 0 5 10 15 20 pretty(1:20, n = 2) # 0 10 20 pretty(1:20, n = 10) # 0 2 4 ... 20 for(k in 5:11) { cat("k=", k, ": "); print(diff(range(pretty(100 + c(0, pi*10^k)))))} ## more bizarre, when min(x) == max(x): pretty(pi) add.names < function(v) { names(v) < paste(v); v} utils::str(lapply(add.names(10:20), pretty)) utils::str(lapply(add.names(0:20), pretty, min.n = 0)) sapply( add.names(0:20), pretty, min.n = 4) pretty(1.234e100) pretty(1001.1001) pretty(1001.1001, shrink.sml = 0.2) for(k in 7:3) cat("shrink=", formatC(2^k, width = 9),":", formatC(pretty(1001.1001, shrink.sml = 2^k), width = 6),"\n")