How to process local txt/brainflow.csv data

in OpenBCI_GUI
Dear OpenBCI community,
Hi all, I am doing a personal signature work, and I have problems to process local csv/txt files. Right now I can only read the data, but I do not know how to visualize the data, or do FFT to the data. I have searched for over 2 days and only found EEG Hacker put forward his solution to txt data, however, I cannot run his code due to some bugs that I cannot solve. Please let me know if there is any other post of how to process txt/csv data, and thank you so much!