There are different methods for the normalization. If it is for visual graphs, you can divide it into bands which can be defined in realtime, so you'd write a program which presents visual graphs of frequency A->B where A and B can be changed by the user. That helps you see display it graphically. using X and Y axis of the mouse to define A->B bandwidth. Then you can scan backwards and forwards through all the frequencies interactively while also normalizing them, using an amplitude meter beside the graph, to say the overall max amplitude of that bandwidth.
If the normalizing process is for high fidelity, balanced post processing and statistics, then it's best to find the highest value at any frequency and divide all the pixel amplitudes by that high value so that it equals 1.