VFChart_Server.class

global class VFChart_Server{
    public Opportunity opportunity {get;set;}

    public VFChart_Server(ApexPages.standardController std){
      opportunity=(Opportunity)std.getRecord();
    
    }
    public List<PieWedgeData> getPieData() {
        List<PieWedgeData> data = new List<PieWedgeData>();
        data.add(new PieWedgeData('Jan', 30));
        data.add(new PieWedgeData('Feb', 15));
        data.add(new PieWedgeData('Mar', 10));
        data.add(new PieWedgeData('Apr', 20));
        data.add(new PieWedgeData('May', 20));
        data.add(new PieWedgeData('Jun', 5));
        return data;
    }

    // Wrapper class
    public class PieWedgeData {

        public String name { get; set; }
        public Integer data { get; set; }

        public PieWedgeData(String name, Integer data) {
            this.name = name;
            this.data = data;
        }
    }
    
    @RemoteAction
    global static void saveChartAsAttachment(String oppId,String attachmentBody){
       system.debug('@@@saveChartAsAttachment  oppId ='+ oppId);
       List<Attachment> listAttachment =[Select Name,Id,Body from Attachment Where ParentId=:oppId];
       Attachment att;
       if(listAttachment.size()>0){
          att =listAttachment[0];
          att.Body = EncodingUtil.base64Decode(attachmentBody);
          update att;
       
       }
       else{
       
           att = new Attachment();
           att.Name='spiderChart_VF';
           att.ParentId=oppId;
           att.ContentType='image/png';
           att.Body = EncodingUtil.base64Decode(attachmentBody);
           insert att;
    
       }
       
      
    
    
    }
}

Thanks for sharing this code. It's very helpful.
Can you please guide me how to use the saved attachment via apex and render it in Visualforce.