Units, plotting and empty plots

maybe I'm completely newbie but I can't understand the behavior of units and the command plot.

I'm doing some easy plotting of the state equation for gasses:

R =  Quantity[0.0831, ("Bars" "Liters")/("Kelvins" "Moles")];
a =  Quantity[3.658, ("Liters")^2]* Quantity[1, ("Bars")/("Moles")^2]  
b = Quantity[0.0429, ("Liters")/("Moles")]
PVW[V_, T_, N_] := ((N*R*T)/(V - N*b)) - (+a*(N^2/V^2));

I then test that units are ok:

test = PVW[Quantity[0.6, "Liters"], Quantity[270, "Kelvins"], Quantity[1, "Moles"]];
UnitSimplify[test]

And I get my pressure back :

Quantity[30.1135, "Bars"]

Now I want my plot so I do :

Plot[PVW[V, Quantity[330, "Kelvins"], Quantity[1, "Moles"] ], {V, 
Quantity[0.06, "Liters"] , Quantity[0.6, "Liters"]},
Frame -> True, 
GridLines -> None,
LabelStyle -> {FontFamily -> "Helvetica", FontSize -> 15},

FrameLabel -> {"Volume", "Pressure"}]

If I evaluate that I get no errors, no nothing but a empty plot:

What is going on?