pretty                 package:base                 R Documentation

Pretty Breakpoints

Description:

     Compute a  sequence of about `n+1' equally spaced nice 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, 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:

       x: numeric vector

       n: 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'.

Details:

     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'.

     .........

Examples:

     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}
     str(lapply(add.names(-10:20), pretty))
     str(lapply(add.names(0:20),   pretty, min = 0))
     sapply(    add.names(0:20),   pretty, min = 4)

     pretty(1.234e100)
     pretty(1001.1001)
     pretty(1001.1001, shrink = .2)
     for(k in -7:3)
       cat("shrink=",formatC(2^k,wid=9),":",
           formatC(pretty(1001.1001, shrink = 2^k), wid=6),"\n")

