Just found out I'm underpaid by $15k. How common is this?

Been working as a journeyman electrician for 3 years making $28/hr. Just found data on TradeWages.com showing the average pay in my area is $36/hr.
Talked to my boss about a raise and he said "that's what apprentices make, not journeymen." But the data was for journeymen.

How many of you are getting lowballed? Feels like companies bank on us not knowing our worth

Anyone else use salary data to negotiate? Did it work?