Common.Gradient
Defines a gradient - linear or radial. For example:
var linearGradient = {
     type: 'linearGradient',
     x0: 0,
     y0: 0,
     x1: 0,
     y1: 1,
     colorStops: [{ offset: 0, color: 'black' },
                  { offset: 1, color: 'white'}]
};
var radialGradient = {
     type: 'radialGradient',
     x0: 0.5,
     y0: 0.5,
     x1: 0.5,
     y1: 0.5,
     r0: 0,
     r1: 1,
     colorStops: [{ offset: 0, color: 'black' },
                  { offset: 1, color: 'white'}]
};