Implementation of R's pretty function in Javascript

r_pretty.js

/**
 * Generate a "pretty" set of ticks (multiples of 1, 2, or 5 on the same order of magnitude for the range)
 * Based on R's "pretty" function:
 * https://github.com/wch/r-source/blob/b156e3a711967f58131e23c1b1dc1ea90e2f0c43/src/appl/pretty.c
 * Arguments:
 *  range (required) :: Two-element array representing the range for the ticks to cover
 *  n     (optional) :: A "target" number of ticks; will not necessarily be the number of ticks you get (default: 5)
 *  internal_only (optional) :: Boolean for whether to only return ticks inside the provided range (default: false)
 */
function pretty(range, n, internal_only){
  if (typeof n == "undefined" || isNaN(parseInt(n))){
    n = 5;
  }
  n = parseInt(n);
  if (typeof internal_only == "undefined"){
    internal_only = false;
  }

  var min_n = n / 3;
  var shrink_sml = 0.75;
  var high_u_bias = 1.5;
  var u5_bias = 0.5 + 1.5 * high_u_bias;
  var d = Math.abs(range[0] - range[1]);
  var c = d / n;
  if ((Math.log(d) / Math.LN10) < -2){
    c = (Math.max(Math.abs(d)) * shrink_sml) / min_n;
  }
  
  var base = Math.pow(10, Math.floor(Math.log(c)/Math.LN10));
  var base_toFixed = 0;
  if (base < 1){
    base_toFixed = Math.abs(Math.round(Math.log(base)/Math.LN10));
  }

  var unit = base;
  if ( ((2 * base) - c) < (high_u_bias * (c - unit)) ){
    unit = 2 * base;
    if ( ((5 * base) - c) < (u5_bias * (c - unit)) ){
      unit = 5 * base;
      if ( ((10 * base) - c) < (high_u_bias * (c - unit)) ){
        unit = 10 * base;
      }
    }
  }
  
  var ticks = [];
  if (range[0] <= unit){
    var i = 0;
  } else {
    var i = Math.floor(range[0]/unit)*unit;
    i = parseFloat(i.toFixed(base_toFixed));
  }
  while (i < range[1]){
    ticks.push(i);
    i += unit;
    if (base_toFixed > 0){
      i = parseFloat(i.toFixed(base_toFixed));
    }
  }
  ticks.push(i);

  if (internal_only){
    if (ticks[0] < range[0]){ ticks = ticks.slice(1); }
    if (ticks[ticks.length-1] > range[1]){ ticks.pop(); }
  }
  
  return ticks;
}