Grapher Calculator is the best graphing calculator for WP.
You can find it here: http://windowsphone.com/s?appid=fd1e8989-f094-4b3d-ba87-d76758b8a023

It supports implict plot now. That means you can plot function like:
x^2+y^2=4 or x^(sin(x*y)+tan(x)=x^y+8

Below is its introduction:
★★★ Top 2 paid app in marketplace of USA!!! ★★★
☆☆☆ Fully support WP8 and all resolution!!! ☆☆☆
★★★ The ONLY app supports implicit plot!!! ★★★

what's new in v1.4:
★ support implicit plot, such as x^2+y^2=4 or e^(sin(x*y))+2*x*y^3=8*x*cos(x+y)
★ modify the icon to be in accord with Metro UI
★ improve the preference when parse expression
★ add some tips when you first use some feature
★ add my twitter account(@wchao911) to the about page

new features will be added in v1.5:
★ math solver

Features:
- includes a powerful and easy to use scientific calculator
- save calculation history
- radians and degrees angle modes
- cartesian, polar and parametric functions support
- pinch to zoom and drag for scrolling the graph in real time
- unlimited amout of equations can be added and plotted simultaneously
- customizable graph colors
- function tracing in cartesian
- graph view supports both portrait and landscape device orientation
- functions: sin, cos, tan, log10, ln, sinh, cosh, tanh, asin, acos, atan, asinh, acosh, atanh, √, │√, rand, abs, sgn, fac, sfac, round, fpart, ceil, floor
Feel free to contact me if you have any question