These functions are used to make expressions for common chemical system variables that can be used in plot legends.
lNaCl describe the molality of NaCl, and lS the total molality of sulfur.
lT and lP describe the temperature and pressure.
lTP describe the temperature and pressure together, separated by a comma.
The above functions return language objects, which can be combined with lex to make an expression that when used in legend appears on multiple lines.