While Obamacare isn't a tax law per se, the US Supreme Court did uphold the validity of the individual coverage mandate penalty on the basis that it was no different than a tax. Chief Justice Roberts had to classify it as a tax to uphold its constitutionality under the Commerce Clause even though the Obama administration vehemently denied it was a tax. 
https://www.forbes.com/sites/ilyashapiro/2014/05/12/the-obamacare-tax-that-chief-justice-roberts-invented-is-still-unconstitutional/#7008d56622b7