It's listed because the graph is a list of "US Drug Overdoses" and cannabis is a widely used drug in the US. Appropriately they show cannabis to be causing zero deaths per year as you and I both know to be true.
I am interested to know why tobacco was left off. I know people don't OD on tobacco either, but since it is also a widely used drug it seems to merit inclusion for the same reason that I think cannabis was included.
If they kept it off b/c you can't OD on tobacco, then in that case you are correct they should have also left cannabis off. Both can kill you down the line or indirectly kill you, but neither will kill you via an "overdose" as this chart is measuring.