How to write a GUI editor for Graph or Tree structures

You are wrong dude. Everything you see in UnityEditor must have code somewhere. Your MecanimEditor is in namespace UnityEditor.Graphs.AnimationStateMachine.

Extend GraphGUI found in UnityEditor.Graphs. This class is responsible for drawing graph.

using System;
using UnityEditor;
using UnityEngine;
using UnityEditor.Graphs;
using System.Collections.Generic;

namespace ws.winx.editor.components
{
 public class GraphGUIEx:GraphGUI{


 }

}

Create new EditorWindow.

public class GraphEditorWindow : EditorWindow
 { 
  static GraphEditorWindow graphEditorWindow;
  Graph stateMachineGraph;

  GraphGUIEx stateMachineGraphGUI;

  [MenuItem("Window/Example")]
  static void Do ()
  {
   graphEditorWindow = GetWindow<grapheditorwindow> ();
  }

....

Create Graph structure. It will contains nodes and edges between nodes.

stateMachineGraph = ScriptableObject.CreateInstance<Graph> ();
    stateMachineGraph.hideFlags = HideFlags.HideAndDontSave;

                 //create new node
    Node node=ScriptableObject.CreateInstance<Node>();
    node.title="mile2";
    node.position=new Rect(400,34,300,200);


    node.AddInputSlot("input");
    start=node.AddOutputSlot("output");
    node.AddProperty(new Property(typeof(System.Int32),"integer"));
    stateMachineGraph.AddNode(node);

//create new node
    Node node=ScriptableObject.CreateInstance<Node>();
    node.title="mile";
    node.position=new Rect(0,0,300,200);

    Slot end=node.AddInputSlot("input");
    node.AddOutputSlot("output");
    node.AddProperty(new Property(typeof(System.Int32),"integer"));
    stateMachineGraph.AddNode(node);

//create edge
    stateMachineGraph.Connect(start,end);

graphGUI = ScriptableObject.CreateInstance<GraphGUIEx>();
graphGUI.graph = graph;

Draw Graph.

void OnGUI ()
  {

   if (graphEditorWindow && stateMachineGraphGUI != null) {
    stateMachineGraphGUI.BeginGraphGUI (graphEditorWindow, new Rect (0, 0, graphEditorWindow.position.width, graphEditorWindow.position.height));
               stateMachineGraphGUI.OnGraphGUI ();


    stateMachineGraphGUI.EndGraphGUI ();

   }
  }

Override NodeGUI or EdgeGUI for more styling and drawing control. Copy paste code from UnityEditor.Graphs.AnimationStateMachine.GraphGUI styling done in NodeGUI and EdgeGUI.

I was not asking "for a find a tool, library or favorite off-site resource". I would like to know how reproduce a Mecanim like graph editor using Unity3D API or some available components provided by the engine itself (sorry if the question wasn't clear).

Here's my answer:

No, there's isn't an available component usable as is to draw that kind of graph, but it's quite easy to write your own. Here's a snippet with a simple example using draggable GUI.Window to represent nodes and Handles.DrawBezier to draw the edges:

public class GraphEditorWindow : EditorWindow
{
    Rect windowRect = new Rect (100 + 100, 100, 100, 100);
    Rect windowRect2 = new Rect (100, 100, 100, 100);


    [MenuItem ("Window/Graph Editor Window")]
    static void Init () {
        EditorWindow.GetWindow (typeof (GraphEditorWindow));
    }

    private void OnGUI()
    {
        Handles.BeginGUI();
        Handles.DrawBezier(windowRect.center, windowRect2.center, new Vector2(windowRect.xMax + 50f,windowRect.center.y), new Vector2(windowRect2.xMin - 50f,windowRect2.center.y),Color.red,null,5f);
        Handles.EndGUI();

        BeginWindows();
        windowRect = GUI.Window (0, windowRect, WindowFunction, "Box1");
        windowRect2 = GUI.Window (1, windowRect2, WindowFunction, "Box2");

        EndWindows();

    }
    void WindowFunction (int windowID) 
    {
        GUI.DragWindow();
    }
}