How to Create a Better Calculator - User Inputs --> Calculations --> Graph Output

I’m new to the forums so sorry if this has been discussed, but hoping you wonderful people can point me in the right direction.

I’ve created some calculators for my website that utilize a microsoft excel add-in. The users can input their asset allocation in the black boxes and it outputs a graph based on the users input.

I’d like to get away from a simple excel add-in and move to something like a form where the user selects an asset from a list, sets their allocation %, and then clicks submit. Then some calculations are done on the back end and a graph/data is presented to the user. Ideal setup would be similar to a website called portfoliovisualizer.

A screenshot of the Input and Submission:

My question. What is the best way to go about doing this as a novice? What tools would I use and are there any good resources to learn how to use them?

My website currently runs on wordpress. I’m open to a paid solution, but would prefer free/cheap.

Thank you!