Stretch available color range to be used based on the values

stretch_colors.js

// Specs:
// - As a default, the colour range should be scaled so, that at least 70% (..ish)  of the colour range is available for the data shown. 
// Example: we have a map with lowest positive correlation at r=0.85 and highest at r=0.99 while the negative correlations span from r=-0.2 to r=-0.35. The desired colour range would then be as follows:
// 100% red at r=0.99, 
// 30% red at r=0.85, 
// white at r=0, 
// 30% blue at r=-0.2 and 
// 100% blue at r=-0.35.

function CustomScale() {
  var obj = {},
  priv = {
    coordinates: [],
    interpolated: {},
    lower: {
      min: null,
      max: null
    },
    upper: {
      min: null,
      max: null
    },
    constant: {
      lower: null,
      middle: null,
      upper: null,
      threshold: 0.3
    }
  };

  function getName(coord, reverse) {
    if(reverse) {
      return [coord.y, coord.x].join("|");
    } else {
      return [coord.x, coord.y].join("|");
    }
  }

  function computeInterpolated() {
    var thre = priv.constant.threshold,
    correlation;
    _.each(priv.coordinates, function(coord) {
      correlation = coord.corr;
      if(correlation > 0) {
        priv.interpolated[getName(coord)] = (thre) + (1-thre) * ( correlation - priv.upper.min ) / (priv.upper.max - priv.upper.min);
      } 
      else if(correlation < 0) {
        priv.interpolated[getName(coord)] = -(thre) - (1-thre) * ( correlation - priv.lower.min ) / (priv.lower.max - priv.lower.min);
      }
    });
  }

  function computeMaxMin() {
    var lowerExtent = d3.extent(priv.coordinates, function(d) {
      if(d.corr > 0) { return undefined; }
      return d.corr;
    });
    var upperExtent = d3.extent(priv.coordinates, function(d) {
      if(d.corr < 0) { return undefined; }
      return d.corr;
    });

    priv.lower.min = _.isUndefined(lowerExtent[0]) ? priv.constant.lower : lowerExtent[0];
    priv.lower.max = _.isUndefined(lowerExtent[1]) ? priv.constant.upper : lowerExtent[1];
    priv.upper.min = upperExtent[0];
    priv.upper.max = upperExtent[1];
  }

  obj.lower = function(x) {
    if(!arguments.length) { return priv.constant.lower; }
    priv.constant.lower = x;
    return obj;
  };

  obj.middle = function(x) {
    if(!arguments.length) { return priv.constant.middle; }
    priv.constant.middle = x;
    return obj;
  };

  obj.upper = function(x) {
    if(!arguments.length) { return priv.constant.upper; }
    priv.constant.upper = x;
    return obj;
  };

  obj.threshold = function(x) {
    if(!arguments.length) { return priv.constant.threshold; }
    priv.constant.threshold = x;
    return obj;
  };

  // get color value
  obj.color = function(obj) {
    var red, green, blue,
    interp = priv.interpolated[getName(obj)];
    interp = _.isUndefined(interp) ? priv.interpolated[getName(obj, true)] : interp;

    if(interp > 0) {
      // upper, red
      red = 255;
      green = 255 * (1 - interp);
      blue = 255 * (1 - interp);
    } else {
      // lower, blue
      red = 255 * (1 + interp);
      green = 255 * (1 + interp);
      blue = 255;
    }
    return d3.rgb(red, green, blue).toString();
  };

  obj.coordinates = function(x) {
    if(!arguments.length) { return priv.coordinates; }
    priv.coordinates = x;
    computeMaxMin();
    computeInterpolated();
    return obj;
  };

  return obj;
}


var scale = new CustomScale()
                .lower(-1)
                .middle(0)
                .upper(1)
                .threshold(0.25);

// call to get value
var color = scale.color();