anchor.diff:      assert(dataPoint['point'].length == 2);
       assert(dataPoint.containsKey('value'));
       assert(dataPoint.containsKey('color'));
  +
  +    minX = dataPoint['point'][0] < minX ? dataPoint['point'][0] : minX;
  +    minY = dataPoint['point'][1] < minY ? dataPoint['point'][1] : minY;
  +    maxX = dataPoint['point'][0] > maxX ? dataPoint['point'][0] : maxX;
  +    maxY = dataPoint['point'][1] > maxY ? dataPoint['point'][1] : maxY;
  +  }
  +
  +  for (Map dataPoint in data) {
       // Color can be an int (the format of the image package for Dart)
       // or a (hex) string.
       String color;
  
positive.code: 
    // Record maximum values of x and y which represent the width and height for
    // background necessary for zooming and panning.
    num maxX, maxY;
    maxX = maxY = double.NEGATIVE_INFINITY;
  
    for (Map dataPoint in data) {
  
negative.code:     // {'point': [x, y],
      //  'value': v,
      //  'color': c,
      //  'uri': path,
      //  ...
      // }
      assert(dataPoint.containsKey('point'));
  