Chart.screenPixelsToPlottingPixels Method
        
 Gets the plotting location in coordinates used by chart axes for the specified screen pixel coordinates,
 which may be different where X and Y axes are inverted or for circular charts.
 
         
        Parameters
        
            - xPixel
            
 
            - 
                
Type: Number - May be null
The number of pixels to the right of the left edge of the chart or its border. 
            - yPixel
            
 
            - 
                
Type: Number - May be null
The number of pixels below the top edge of the chart or its border. 
        
        Return Value
        
Type: Object
        
An object with properties:
     x   [type: Number]  - The plotting pixel coordinate for the X axis.
     y   [type: Number]  - The plotting pixel coordinate for the Y axis.