Support equations in EasyChart calculated pens
Allow EasyChart calculated pens to accept basic equations like:
historicalPen * 5 / 8 + 32
This would allow converting a historical pen stored in degrees Celsius to Fahrenheit and other similar units of measure conversions. This in turn allows displaying a trend in different units of measure than it was recorded in rather than recording the same data twice (once in each unit of measure).
A distant second option would be to allow basing a calculated pen on another calculated pen. This would allow building the conversion in steps with each calculated pen. (Presently, calculated pens cannot be based on calculated pens, preventing this.)
Oops, make that equation: historicalPen * 8 / 5 + 32