EPPlus VS ScottPlot

Like others, I recommend using the ImportExcel module. It uses and wraps EPPlus Software (EPPlus - Create advanced Excel spreadsheets using .NET (GitHub Project Old / New)) which is a DLL that is included with the ImportExcel PowerShell Module.

Their documentation looks okay. I doubt you're going to get many people familiar with a specific library here.

If you're copypasting stuff from excel, another approach I used in a different project is this package: https://github.com/EPPlusSoftware/EPPlus - which works in Blazor. You'd just let users drag/drop or upload their excel and import it, instead of copypasting

There is one Excel Library that I would recommend using in C# and that EPPlus. Its great and its free for non-commercial use. The API is simple to use and its got great documentation.

There is a Getting Started section in GitHub and complete API documentation on the home page

I've personally had a great experience working with EPPlus. It has both commercial and non-commercial/free licenses: https://epplussoftware.com/

The pull request from which this code is based on: https://github.com/ScottPlot/ScottPlot/pull/2221

ScottPlot is another .NET option besides OxyPlot and LiveChartsv2. There are a few libs that wrap matplotlib from Python like MatplotlibCS if that's your thing. You could also WebView2 and d3.js for an offline app. It really depends on what you're producing charts for: a highly stylized figure might not be appropriate for an academic publication, interactivity may be important, a very specific type of chart out-of-the-box might be important, etc.

I know you said no 3rd party controls but https://scottplot.net/ is a free and open-source plotting library for .NET which you can use from WPF. The Github page is here https://github.com/ScottPlot/ScottPlot

I dont know what type of visualisations you need but if you are doing a simple WinForms app you cant beat ScottPlot

I was mostly wondering about the charting library itself, in isolation of QuestPDF. ScottPlot is the other one I was considering besides microcharts. Since you mentioned using QuestPDF in production for reporting, I was wondering if perhaps you also used any of the OSS charting libraries in building those reports and if you have any insights you can share. Docs and demos are a good start when picking one, but lived experience is very valuable.