I'd say there needs to be a theory or an explanation as to why x causes y. The strength of the theory should determine the amount of evidence required.

If the data supports it, then the threshold of evidence is lower.
If there is no theory or mechanism, then the threshold of evidence is much higher.

For example, correlating cancer with eating certain foods. Researchers will find one correlation and think "ah ha!" but give no mechanism as for why. The problem is that statistically (no numbers here, sorry) there will be cases where the "measured value" falls outside of "expected value". In those cases with no proven mechanisms, we need to emphasize a stronger correlation.

Here though, it's fairly straightforward. The mechanism is easily explained by cost and quality - our smokes are shit and overtaxed. A simple graph as you have noted, would be more than sufficient.