The following tasks outline this approach:
1. Create a dll [CustomCodeSSRS]

2. Define an entry point there:

using System;
using System.Collections.Generic;
using System.Text;
using Nevron.GraphicsCore;
using Nevron.Chart;
using Nevron.ReportingServices;

namespace SSRSCustomCode
{
    public class Class1
    {
        /// <summary>
  /// Main entry point
  /// </summary>
  /// <param name="context"></param>
  public static void RSMain(NRSChartCodeContext context)
  {
            // Do something here just like in SSRS
  }
    }
}
3. Call that entry from the SSRS designer:

using System;
using System.Drawing;
using Nevron.GraphicsCore;
using Nevron.Chart;
using Nevron.ReportingServices;
using SSRSCustomCode;

namespace MyNamespace
{
 /// <summary>
 /// Sample class
 /// </summary>
 public class MyClass
 {
  /// <summary>
  /// Main entry point
  /// </summary>
  /// <param name="context"></param>
  public static void RSMain(NRSChartCodeContext context)
  {
   Class1.RSMain(context);
  }
 }
}

4. Reference the CustomCodeSSRS dll in the SSRS designer "Build \ Referenced Assemblies" add "[AppDir]\SSRSCustomCode.dll"
5. Deploy the CustomCodeSSRS dll to the SSRS designer and report server folders. For example:
C:\Program Files (x86)\Microsoft Visual Studio 8\Common7\IDE\PrivateAssemblies\
c:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportServer\bin

You should be able to attach a debugger to Visual Studio and debug from there. Hope this helps - let us know if you meet any problems or have any questions.