I see Slate has apparently shifted the "mass shooting" definition back again.
By the definition in your first chart (the Mass Shootings Calendar), the Monash University shooting would be a "mass shooting." Two were killed, four injured. The Hectorville Siege was 2011. Three killed, four injured. The Hunt family killing, 2014. Four killed, plus the shooter.
Just remember, you moved the goalposts.
The above chart might be more significant if the law had been in 1991. But I see a steady drop with little change (except for the '98 outlier) in the slope until it flattens out in about '05. Other similar charts show the same thing. If it wasn't for '98, the ones with lines would show little evidence that anything had happened in '96.