Gnuplot dies when fed hundreds of megabytes of data. At some point you have to start making your own tools. Here's an example:

http://kmkeen.com/rtl-power/tholin_rtlsdr.png

Every pixel is a datapoint. There are only 1.5M values in that render (3-tuples of time, frequency and intensity) but the resolution of that chart is pretty low. People casually do charts a hundred times larger. Then you start running into other problems, like sharing the images. Firefox won't display images wider/taller than 32k px for example.