/**
 * Returns a linear color ramp from the specified <tt>start</tt> color to the
 * specified <tt>end</tt> color. The color arguments may be specified either as
 * <tt>string</tt>s or as {@link pv.Color}s. This is equivalent to:
 *
 * <pre>    pv.Scale.linear().domain(0, 1).range(...)</pre>
 *
 * @param {string} start the start color; may be a <tt>pv.Color</tt>.
 * @param {string} end the end color; may be a <tt>pv.Color</tt>.
 * @returns {Function} a color ramp from <tt>start</tt> to <tt>end</tt>.
 * @see pv.Scale.linear
 */
pv.ramp = function(start, end) {
  var scale = pv.Scale.linear();
  scale.range.apply(scale, arguments);
  return scale;
};