D3 elastic scale closure

elasticScale.js

// a wrapper around d3's linear scale;
// extreme inputs (outside domain) push the domain while maintaining a fixed extent
function elasticScale() {
  var sensitivity = 27,
      origin = 0,
      domain = [origin - sensitivity/2, origin + sensitivity/2],
      scale = d3.scale.linear()
        .domain(padDomain(domain))
        .range([0, 0, 1, 1])
        .clamp(true);

  function eScale(theta) {
    // the crux of it

    if(theta < domain[0]) {
      domain[0] = theta;
      domain[1] = theta + sensitivity;
      scale.domain(padDomain(domain));
    } else if (theta > domain[1]) {
      domain[1] = theta;
      domain[0] = theta - sensitivity;
      scale.domain(padDomain(domain));
    }

    return scale(theta);
  }

  eScale.sensitivity = function(value) {
    if (!arguments.length) return sensitivity;
    sensitivity = value;
    domain = [origin - sensitivity/2, origin + sensitivity/2];
    scale.domain(padDomain(domain));
    return etScale;
  };

  function padDomain(domain) {
    return [
    domain[0],
    domain[0] + (1/3) * (domain[1] - domain[0]),
    domain[0] + (2/3) * (domain[1] - domain[0]),
    domain[1]
    ];
  }

  return eScale;
}