Xof1986, just to clarify, yes you can use standard units that aren't listed in the drop-down and combine units. See the documentation below that I have copied from the help link in Mango (the blue help icon) in particular the two links to SI and non-SI units. By combine I mean that you can write V*s or m³/s

You can specify a Unit for a point. Type in the symbol for the unit e.g. kg, L, mA, Hz, lb. Note that for some units there are multiple ways of entering the unit, e.g. for degrees celsius you can enter °C or Celsius, for ohms you can use Ω or Ohm. Also remember that there are sometimes multiple definitions of a unit, e.g. there are metric tonnes (t), short tons (ton_us) and long tons (ton_uk). You can use all SI prefixes on SI units, e.g. you can enter W for watt and also µW (microwatt), MW (megawatt) etc.
For SI Units see the list here here.
For NonSI Units see the list here.
Note that there are also some units internally defined in this system:

btu - British thermal unit
thm - Thermal Unit (BTU * 100000)
psi - Pounds per square inch
rpm - Revolutions per minute
ppm - Part per million
ppb - Part per billion
There are also some aliases for common units defined here:
Celsius - °C
Fahrenheit - °F
The Displayed unit specifies which unit the renderer should convert to for display (if supported by the renderer). Input format is the same as for Unit.

The Integral unit used in the display of statistic and specifies which unit the integrated total should be displayed in. Input format is the same as for Unit.