Hi Ranjith,
This is one of the areas where we do not currently have enough content – that explains how to create charts for XLSX using the Open XML SDK.
Are you comfortable, in general, with creating and modifying Open XML documents using the Open-Xml-Sdk? You need to be familiar with creating markup and creating parts, where the parts have a given content type, and creating relationships between parts, where the relationships have the appropriate relationship type.
The approach that you need to take is to create a spreadsheet that does not have the desired text box, copy it, then in the copy, add the text box, then use the Open XML SDK Productivity Tool to determine the differences between the two. This will teach you about the markup you need to create. See the following screen-cast:
Screen-Cast: How to Research Open XML Markup
Beyond that, I recommend that you watch the various screen-casts that are an introduction to Open XML:
See the screen-cast series in the sidebar on this blog.