CSHARP

unity inspector draw line

   //============================================================
   void GuiLine( int i_height = 1 )
   {
       Rect rect = EditorGUILayout.GetControlRect(false, i_height );
       rect.height = i_height;
       EditorGUI.DrawRect(rect, new Color ( 0.5f,0.5f,0.5f, 1 ) );
   }

unity custom editor draw line in scene

 Handles.DrawLine(pos1, pos2);