How to write a desktop application like Excel, C#, cross-platform?

Question

excel

It is necessary to write a program where the user will enter a lot of numerical information and it needs to be processed and made graphs.
First of all, speed and ease of development are needed, I know C # and Java, I "heard" about Python

Answer 1

The choice of language in this case is primarily determined by the libraries that can be used, but this is not clear without knowing the task.
Why not use ready-made libraries in one way or another? There are a couple of ready-made solutions. for example ReoGrid (this is for C #), which will provide WYSIWYG for editing sheets and graphs and save it to a file. But you can make a "hybrid", for example, write your own WYSIWYG, but to store files, use a model from the NPOI or POI library that works with *.XLS and *.XLSX, you will get compatibility with Excel files.
It is not clear why write your own solution at all, why not limit yourself to a macro, or ActiveX in order to work with the Excel COM model and embed it WYSIWYG into your window (this is also possible).
Cross-platform should also be clarified.
Windows/OSX or Windows/Linux?