How to round up to the nearest 10 (or 100 or X)?

The plyr library has a function round_any that is pretty generic to do all kinds of rounding. For example

library(plyr)
round_any(132.1, 10)               # returns 130
round_any(132.1, 10, f = ceiling)  # returns 140
round_any(132.1, 5, f = ceiling)   # returns 135

If you just want to round up to the nearest power of 10, then just define:

roundUp <- function(x) 10^ceiling(log10(x))

This actually also works when x is a vector:

> roundUp(c(0.0023, 3.99, 10, 1003))
[1] 1e-02 1e+01 1e+01 1e+04

..but if you want to round to a "nice" number, you first need to define what a "nice" number is. The following lets us define "nice" as a vector with nice base values from 1 to 10. The default is set to the even numbers plus 5.

roundUpNice <- function(x, nice=c(1,2,4,5,6,8,10)) {
    if(length(x) != 1) stop("'x' must be of length 1")
    10^floor(log10(x)) * nice[[which(x <= 10^floor(log10(x)) * nice)[[1]]]]
}

The above doesn't work when x is a vector - too late in the evening right now :)

> roundUpNice(0.0322)
[1] 0.04
> roundUpNice(3.22)
[1] 4
> roundUpNice(32.2)
[1] 40
> roundUpNice(42.2)
[1] 50
> roundUpNice(422.2)
[1] 500

[[EDIT]]

If the question is how to round to a specified nearest value (like 10 or 100), then James answer seems most appropriate. My version lets you take any value and automatically round it to a reasonably "nice" value. Some other good choices of the "nice" vector above are: 1:10, c(1,5,10), seq(1, 10, 0.1)

If you have a range of values in your plot, for example [3996.225, 40001.893] then the automatic way should take into account both the size of the range and the magnitude of the numbers. And as noted by Hadley, the pretty() function might be what you want.

The round function in R assigns special meaning to the digits parameter if it is negative.

round(x, digits = 0)

Rounding to a negative number of digits means rounding to a power of ten, so for example round(x, digits = -2) rounds to the nearest hundred.

This means a function like the following gets pretty close to what you are asking for.

foo <- function(x)
{
    round(x+5,-1)
}

The output looks like the following

foo(4)
[1] 10
foo(6.1)
[1] 10
foo(30.1)
[1] 40
foo(100.1)
[1] 110

If you add a negative number to the digits-argument of round(), R will round it to the multiples of 10, 100 etc.

    round(9, digits = -1) 
    [1] 10    
    round(89, digits = -1) 
    [1] 90
    round(89, digits = -2) 
    [1] 100

How about:

roundUp <- function(x,to=10)
{
  to*(x%/%to + as.logical(x%%to))
}

Which gives:

> roundUp(c(4,6.1,30.1,100.1))
[1]  10  10  40 110
> roundUp(4,5)
[1] 5
> roundUp(12,7)
[1] 14