I track my mobile data monthly and have been doing this since early 2010.
You can certainly see where the smartphone data picked up in April 2011 when I got a Samsung Galaxy S. My wife's data picked up later that year when she got an iPhone.
The bright green bars are from Android Auto or Car Play. The spikes there are when we are traveling and using both the mapping capabilities and streaming Memphis radio.
AT&T's plans have been outstripping my mobile data usage. For the last 12 months, my average was less than 2GB. The highest month was less than 4GB when we were traveling. My AT&T plan is 35GB with one month rollover.
I regularly review AT&T's plans but lowering the data plan doesn't significantly reduce the monthly cost. Their incentives are to drive customers to unlimited data plans. This is often at the loss of tethering.