I wasn't thinking about the LucidChart per say. I was wondering how an Web based-app could be intergrated into Excel such that a user was prompted to login and then import user specific data directly into a Excel spreadsheet range using an REST API.
Excel has had that capability forever. Excel VBA code had access to XmlHttpRequest as soon as Internet Explorer did, at version 5.0. XHR is a COM object that can be invoked from just about anywhere in Microsoftland. I was doing this in Excel way back in 2000.
Nowadays XHR is a bit limited as far as exception handling and other niceties you'd like for API calling. But Excel VBA can access anything through COM interop, such as a more robust C# library using .NET objects like HttpWebRequest.
Excel also has a notion of web queries, to populate a range by making an HTTP request and scraping a table out of the HTML. I think that existed as long ago as Excel 2000 and was brought out to the UI in 2007. It doesn't play well with websites requiring a login or a nontrivial click path, though. I have vague memories of wrestling with this, through the spectacular method of launching an IE window from Excel and relying on the MSHTML layer to share the login cookies between that and Excel!
Excel supports task pane and content apps. So for a task pane app like this, it would be possible to make the same app work in both Word and Excel by modifying the "Capabilities" part o the manifest file. All that needs to be added is the following tag: <Capability Name="Workbook"/>
If the user experience is appropriate, then apps typically work very well in both Word and Excel without any changes to the application code.
