Optional unit
unit?: "meter" | "degree-celsius" | "degree-fahrenheit" | "kelvin" | "duv" | "volt-dc" | "volt-ac-peak" | "volt-ac-rms" | "amp-dc" | "amp-ac-peak" | "amp-ac-rms" | "hertz" | "ohm" | "watt" | "kilogram" | "square-meter" | "cubic-meter" | "meter-per-second" | "meter-per-second-squared" | "newton" | "joule" | "pascal" | "second" | "degree" | "steradian" | "candela" | "lumen" | "lux" | "ire" | "byte" | "percent" | "millisecond" | "rpm" | "unitless"