In his Mastering JMP demo on tracking and trending manufacturing metrics, JMP Systems Engineer Byron Wingerd @Byron_JMP suggests you need a minimum of 25 points, but charting fewer can give some information. Don Wheeler suggests transforming provides limited information about outliers and presents the problem that transformed units may not be useful to people trying to improve the process, so the analyst must reverse transfrom units for actionable results. (One exception to not transforming may be when you have potency data.)