can we get Custom scroll bar as like image? not for entire diagram like myDiagrame.allowHorizontalScroll = true ; but specific part of the diagram?
If I understand correctly your intent, that is possible but will require programming on your part.
Have you seen http://gojs.net/latest/extensions/ScrollingTable.html ? You will need to extend the implementation of http://gojs.net/latest/extensions/ScrollingTable.js.
Hmmm, actually do you want to scroll the tree of nodes within a group? The ScrollingTable panel is defined to scroll the contents of a Table Panel. You will need to adapt the code to change the opacity of some of the Nodes and Links in the tree and move them all up or down by the height of the node(s).
can i use any plugins? like http://manos.malihu.gr/repository/custom-scrollbar/demo/examples/complete_examples.html
No. For efficiency all GraphObjects are not HTML elements.