pretty: Pretty Breakpoints

Description

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.

Usage

pretty(x, ...)
"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, ...)

Arguments

an object coercible to numeric by as.numeric.

integer giving the desired number of intervals. Non-integer values are rounded down.

min.n

nonnegative integer giving the minimal number of intervals. If min.n == 0, pretty(.) may return a single value.

shrink.sml

positive numeric by a which a default scale is shrunk in the case when range(x) is very small (usually 0).

high.u.bias

non-negative numeric, typically $> 1$. The interval unit is determined as {1,2,5,10} times b, a power of 10. Larger high.u.bias values favor larger units.

u5.bias

non-negative numeric multiplier favoring factor 5 over 2. Default and ‘optimal’: u5.bias = .5 + 1.5*high.u.bias.

eps.correct

integer code, one of {0,1,2}. If non-0, an epsilon correction is made at the boundaries such that the result boundaries will be outside range(x); in the small case, the correction is only done if

eps.correct
       >= 2

...

further arguments for methods.

Details

pretty ignores non-finite values in x.

Let d <- max(x) - min(x) $\ge 0$. 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 $b \le c < 10b$.

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.

.........

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Examples

Run this code

pretty(1:15)          # 0  2  4  6  8 10 12 14 16
pretty(1:15, h = 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 = 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")