code: function getDataContext(ctxOrCanvas) {
    if (ctxOrCanvas instanceof HTMLCanvasElement) {
      return getDataContext(ctxOrCanvas.getContext('2d'));
    }
  
    var ctx = ctxOrCanvas;
    if (stubGetDataContext) {
      return stubGetDataContext(ctx);
    } else {
      for (var i = 0; i < getDataContext.cache.length; i++) {
        var pair = getDataContext.cache[i];
        if (pair[0] == ctx) return pair[1];
      }
      var dtx = new DataContext(ctx);
      getDataContext.cache.push([ctx, dtx]);
      return dtx;
    }
  }
docs: Get a DataContext for the built-in CanvasRenderingContext2D.
  
  This caches DataContexts and facilitates stubbing in tests.
  
  As a convenience, you may pass in a Canvas element instead of a
  CanvasRenderingContext2D. data-canvas will call getContext('2d') for you.