Free Graphing Calculator -- iOS:
https://itunes.apple.com/us/app/free-graphing-calculator/id378009553?mt=8
Android:
https://play.google.com/store/apps/details?id=com.jockusch.freegraphingcalculator
Mac:
https://itunes.apple.com/us/app/free-graphing-calculator-2/
Windows:
https://www.microsoft.com/store/apps/9pgllk5gj04h