Possible incorrect output parameter

[wdevore]

https://github.com/wcharczuk/go-chart/blob/11e380634bd594f5635f2f8e89a6e8c075ec1222/image_writer.go#L34

The current signature can't be type cast, I believe the output return parameter should be:

(*image.RGBA, error)

I changed a copy of the *Image_Writer.go and it works.