There's really no need for that.
set trange [0:1]
phi1(t)=-pi+2*pi*t
phi2(t)=2*pi*t
plot sin(phi1(t)), cos(phi1(t)), title 'circle' \
phi2(t), tan(phi2(t)) title 'tangent'
If multiple parametric functions are defined in a plot, it is only possible to define one same range for all parametric functions that have to be displayed.
So, it would be useful to allow a range for each parametric function, e.g.
set parametric
plot [-pi,pi] sin(t), cos(t), [0,2*pi] t, tan(t)
There's really no need for that.
set trange [0:1]
phi1(t)=-pi+2*pi*t
phi2(t)=2*pi*t
plot sin(phi1(t)), cos(phi1(t)), title 'circle' \
phi2(t), tan(phi2(t)) title 'tangent'