Thank you to 'KeyLo99' for submitting news of the release of his new RTL-SDR based program called rtl_map. rtl_map is a currently a simple app that uses an RTL-SDR to display an FFT frequency graph. It is based on the gnuplot and fftw3 libraries.
Over on our forums KeyLo99 describes the motivation behind the project as mostly being a good reference program for people wanting to learn how to read and process IQ data from the RTL-SDR:
I'm a RTL-SDR researcher and DSP learner currently working on a project for properly figuring RTL2832 and I/Q fundamentals out. The project is about reading raw I/Q samples, processing samples and creating FFT graph from them. I tried to explain what I'm doing in detail with comment lines. I'm hoping that I will be helpful to RTL-SDR beginners with this rtl_map [C] project. Another purpose of the rtl_map project is making a frequency scanner application for signal security researches.