Creates a new DygraphLayout object.

This class contains all the data to be charted.
It uses data coordinates, but also records the chart range (in data
coordinates) and hence is able to calculate percentage positions ('In this
view, Point A lies 25% down the x-axis.')

Two things that it does not do are:
1. Record pixel coordinates for anything.
2. (oddly) determine anything about the layout of chart elements.

The naming is a vestige of Dygraph's original PlotKit roots.

@constructor
function(dygraph) {
  this.dygraph_ = dygraph;
  /**
   * Array of points for each series.
   *
   * [series index][row index in series] = |Point| structure,
   * where series index refers to visible series only, and the
   * point index is for the reduced set of points for the current
   * zoom region (including one point just outside the window).
   * All points in the same row index share the same X value.
   *
   * @type {Array.<Array.<Dygraph.PointType>>}
   */
  this.points = [];
  this.setNames = [];
  this.annotations = [];
  this.yAxes_ = null;

  // TODO(danvk): it's odd that xTicks_ and yTicks_ are inputs, but xticks and
  // yticks are outputs. Clean this up.
  this.xTicks_ = null;
  this.yTicks_ = null;
}