Signal Processing at frequency domain

Is there any C# libraries, Methods, maybe some matlab plugins for C# app. that would help me plot a row signal values in time domain as well as it can transfer it to the frequency domain?

I'm also looking for some way to plot values based on a specific frequency, I though of filtering the input signal before plotting it and now looking for some software filter/plugin/library that can help me.

Basically what I'm trying to do is to manipulate the input raw signal, and do some filtration and FFT and other signal processing functionalists.