Data Entry:

You could give the user a text box and button and then when they push the button parse the text to ensure it's a valid mathematical expression. Alternatively you could build an Equation Editor GUI that forces the user to build an equation step by step so that each step ensures the integrity of the equation. Either way you're then probably going to have to convert this into some object or hierarchy of objects that will execute the equation they've entered.

Graphing:

There are tons of graph controls out there, just Google "C# graph control". Maybe you can even find one that accepts and graphs an equation as is. If not then you'll probably have to run a bunch of numbers through the equation to generate a good number of data points. Any decent graph control should be able to plot a set of data points but if you really want to you could even roll your own graph control to do the job.

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