This chart represents live trade volume data from across the Steam network and annotates it with major events. (Note that many of the spikes are summer/winter Steam sales during which a lot of card trading happens):
Feel free to grab the raw data. Updates daily.TradeTF has been collecting this data since April of 2019. Data from 2013-2018 was provided by BazaarTF. Special thanks to Spyfly from STN.tf for providing 5 months worth of data that I was missing.