One thing to call out on the graph- it's a measure of overdose deaths. About 75% of the deaths were opioid related.
Honestly, I don't think the problem is an solely an increase in the number of addicts. The problem is that they are not getting the lower-quality heroin that they're used to.
Carfentanil is so potent that it is almost always lethal. If morphine has a potency of "1", heroin would be "2", fentanyl would be "100", carfentanil would would be "10,000".
Many opioid addicts are functional addicts. They have jobs where their addiction isn't as obvious- working in nightclubs, gig workers, day laborers, etc. Unemployment may have removed the need to be functional and the mental health stresses that came with the pandemic exacerbated their addiction. Most sources attribute the increase in deaths to fentanyl and carfentanil.
If you've been dealing with a heroin addict, then you know that it doesn't matter where the money is coming from, as long as it's coming in. They'll stay functional to keep an income to pay for their habit. They'll steal from work, from family or wherever they can. So, being unemployed and getting an unemployment check or a stimulus check is no different from getting a paycheck, working day labor for cash or stealing. Money is money.