Here you can find a WPF app with a click once deployment. I use Roslyn script engine to turn C# code into function that is being graphed on a canvas.

This is how a common output looks:

For example you can write something like:

x*x

OR

sin(x*PI)

OR something more complex like:

var flatness = 2;
var period = x * PI;
return sqrt(
    (1 + flatness * flatness)
     /
     (1 + flatness * flatness * cos(period)*cos(period))
) * cos(period)

The code must be valid C#. Here you can see a list of functions you can use, plus anything from the Math object of .NET framework.

And here is a list of functions that I often use when writing games: http://www.bodurov.com/funcs/

You can also use bezier editor to compose complex functions

Find the source code on GIT Hub.