Putting piechart in of Geoserver style?

I reckon that GeoServer layer label style has excellent feature of TextSymbolizer where text label can be positioned relatively on centroid of VISIBLE PART of polygon, as oppose to a geographically fixed position. The GeoServer website states that can be specified in a .

With this kind of placement i also need to visualise piechart. This is where i found difficulties.

Here is my SLD, within the there's a piechart and a .

 xmlns:sld="http://www.opengis.net/sld"
 xmlns:ogc="http://www.opengis.net/ogc"
 xmlns:gml="http://www.opengis.net/gml"
 xmlns:xlink="http://www.w3.org/1999/xlink"
 version="1.0.0">
  

    test
    
      test
      
      
        
          
            
              
                
                  xlink:href="http://chart?cht=p&chd=t:${100 * revenue_percentage},${100 * (1 - revenue_percentage)}&chf=bg,s,FFFFFF00" />
                application/chart
              
              
                
                  20
                  
                    
                      revenue_target
                      2e13

                    
                    60
                  
                
              
            
            
              PIECHART GRAPHIC
            
            

              SansSerif
              14
              normal
              bold
                  
            
            
              1
                        
              

                
                  0.5
                  0.5
                
              
            
          
        
      
    

  

I found that:

1. In order a graphic to show, text must be present in a . And the must not be empty. If is not wanted then the only getaway is to hide it with 0


2. If is not wanted it is good to put a shortest possible string in the (for example a point ".") to make sure that element of the same are drawn.


3. With in , GeoServer GUI complaints when i click the "Validate" button in Style Editor. The Style Editor says while i believe and are optional. However, the style can be submitted and can be use with a WMS Layer without error.


4. The piechart never showed up in WMS, and not even an error message from GeoServer (this is observed using FireBug, the request is fine and nothing is wrong).By the way the expected piechart is below

5. If I replace the piechart in with a PNG image then the PNG Images are drawn nicely in WMS response, off course with in the . The expected placement works. The also works, depending on value of a field. Screenshot below is when i panned slightly to the west.


6. If I replace the with , the piechart shows. Surely if i panned the map, the piechart stay at the same geographic location. Unfortunately the extent in the above images do not cover centroid of polygon, hence the below screenshot uses different extent. Here's the screenshot:

My questions:

1. How to make the piechart in drawn in WMS response?


2. Is this because doesn't like piechart ?

My environment:

• GeoServer 2.5 (+Chart Extension) on Apache Tomcat 6.0.35


• OpenLayers 2.12


• PostgreSQL 9.2.4-1 + PostGIS 2.0.3-2


• Windows Server 2003 32-bit