I would suggest Java for the server-side coding.
While I do not know the specifics of the financial calculations you mentioned, the Java ecosystem is vast. I fully expect you will be able to find libraries to assist with those.
As for accuracy, you will need to avoid floating-point technology, which trades away accuracy for speed of execution. For accurate handling of fractional numbers for matters such as money, Java provides the
You tagged Microsoft Excel. For parsing or generating spreadsheet files there are Java-based libraries such as Apache Poi.
For building the user-interface and app server of web apps in Java, use Vaadin.
Vaadin is free-of-cost and open-source.
As a commercial add-on, the Vaadin Ltd company sells a Vaadin Charts product to render a wide-variety of charts. You can find other charting solutions too.