private static extern int gvRenderData(GVC_t gvc, Agraph_t g, string format, ref IntPtr result, ref uint length);
private GraphVizLib() { gvc = gvContextPlugins(IntPtr.Zero, true); }
private static extern void gvFinalize(GVC_t gvc);
private static extern int gvFreeContext(GVC_t gvc);
private static extern int gvFreeLayout(GVC_t gvc, Agraph_t g);
private static extern int gvLayout(GVC_t gvc, Agraph_t g, string engine);
private static extern string gvcBuildDate(GVC_t gvc);
private static extern string gvcVersion(GVC_t gvc);
private static extern string[] gvcInfo(GVC_t gvc);