You need to think about:
- Where the calculations and visualizations will happen (server or client side)
- What data you have to store (server side)
- Security and financial concerns
The actual code for this will be a mathematical algorithm. It will basically look and work the same no matter which language you use. Even for creating the visual graph the code will be pretty much the same as the basic Graphics libraries are all similar (.Net GDI+, PHP GD, etc.). The difficult part will be writing the correct algorithm (Math!) to handle this accurately, after that the code will be very portable.
If you need to continually save the data in a database, you will need a server running a database. Web applications like this are usually done in either .Net and MS SQL Server or PHP and MySQL. Other than price (PHP/MySQL hosting is cheaper) there should be no practical advantage to either, just what you're most comfortable coding in. Both have tonnes of resources online for learning how to code. (Try pscode.com and Google)
A bigger issue is security. If this data is being saved to a Database you want to make sure it is not vulnerable to SQL-injection or other types of attacks. A compromised database will make this whole thing fall a part.
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EDIT
MySQL is a database, it is for saving data on a server. PHP is a server-side scripting language, it is for handling data on a server and displaying it to the client.
If what you want to do does not need to save data, then this can be handled without your own database. (I had pictured a graph over time, which would need to save previous data.) This would require on-demand third-party Services where the data will come from when the client clicks "Show me Andrew's Financial Thing".
You would still likely want to handle this server-side (PHP), as you will have more security and control over what is displayed. (As opposed to client-side scripting such as Javascript.) The PHP page can display your visualization on regular page, as part of a Facebook app, or for a mobile browser page.
Doing this for an iPhone App will require either a reimplementation in Objective-C of the algorithm and graphics code (it connects to third party services), making an iPhone app that connects to the PHP page and displays that data directly (as if it where a web browser), or a combination of the two (it checks with the php page to get the information but creates its own visualizations.)
For example, (sorry to plug) we recently made an iPhone app that graphs financial information inputted by the client.
http://itunes.apple.com/us/app/assetgraphit/id376400107?mt=8
http://www.makeitsostudios.com/app_assetgraphit.html