# Plotting horizontal line on Manipulate Plot

I have the following code, which outputs a Manipulate style plot. I want to draw a horizontal line on the plot related to q, with equation: y = Roche[\[rho]].

My code is as follows:

Constants  au = QuantityMagnitude[UnitConvert[Quantity[1, "AstronomicalUnit"], "Meters"]];  c = QuantityMagnitude[UnitConvert[Quantity[1, "SpeedOfLight"], "MetersPerSecond"]];  Qpr = 1;  Lsun = QuantityMagnitude[UnitConvert[Quantity[1, "SolarLuminosity"], "Watts"]];  Rsun = QuantityMagnitude[UnitConvert[Quantity[1, "SolarRadius"], "Meters"]];  Msun = QuantityMagnitude[UnitConvert[Quantity[1, "SolarMass"], "Kilograms"]];  G = QuantityMagnitude[UnitConvert[Quantity[1, "GravitationalConstant"], ("Meters"^2*"Newtons")/"Kilograms"^2]];  year = QuantityMagnitude[UnitConvert[Quantity[1, "Years"], "Seconds"]];  Myr = year*10^6;  Gyr = year*10^9;  Mwd = 0.6*Msun;  Cst = 1.27;  U = 1*10^17;   Functions  L[t_] := (3.26*Lsun*(Mwd/(0.6*Msun)))/(0.1 + t/Myr)^1.18;  Roche[dens_] := (0.65*Cst*Rsun*(Mwd/(0.6*Msun))^(1/3))/(dens/3000)^3^(-1);  Papsis[t_] := a[t]*(1 - e[t]);   Radiative Drag  RDdadtR\[Rho]a = -((3*L[t]*Qpr*(2 + 3*e[t]^2))/(c^2*(16*Pi*2000*Rast*a[t]*(1 - e[t]^2)^(3/2))));  RDdedtR\[Rho]a = -((15*L[t]*e[t])/(c^2*(32*Pi*Rast*2000*a[t]^2*Sqrt[1 - e[t]^2])));   Null  RDsolR\[Rho]a = ParametricNDSolveValue[{Derivative[1][a][t] == RDdadtR\[Rho]a, Derivative[1][e][t] == RDdedtR\[Rho]a, a[0] == a0, e[0] == 3/10}, {a, e}, {t, 0, 9*Gyr},      {Rast, \[Rho], a0}];   fRDticks = {{Automatic, Automatic}, {Charting`FindTicks[{0, 1}, {0, 1/Myr}], Automatic}};   Manipulate[Column[{Style["Working Plot", Bold], Plot[fun[func, t]/scale[func], {t, 0, 9*Gyr}, FrameTicks -> fRDticks,       PlotStyle -> {Directive[Blue, Thickness[0.01]]}], Style["Compiled Plot", Bold],      If[comp === {}, Plot[fun[func, t]/scale[func], {t, 0, 9*Gyr}, FrameTicks -> fRDticks, PlotStyle -> {Directive[Blue, Thickness[0.01]]}],       Plot[comp, {t, 0, 9*Gyr}, FrameTicks -> fRDticks, PlotStyle -> {Directive[Blue, Thickness[0.01]]}]]}], {{func, 1}, {1 -> "a", 2 -> "e", 3 -> "q"}},    {{Rast, 0.005}, 0.0001, 0.1, 0.001, Appearance -> "Labeled"}, {{\[Rho], 3000}, 1000, 7000, 50, Appearance -> "Labeled"},    {{a0, 10, "a0 (au)"}, 2, 20, 0.2, Appearance -> "Labeled"}, Button["Append", AppendTo[comp, fun[func, t]]], Button["Reset", comp = {}],    TrackedSymbols -> {func, Rast, \[Rho], a0}, Initialization :> {comp = {}, fun[sel_, t_] := Switch[sel, 1, RDsolR\[Rho]a[Rast, \[Rho], a0*au][[1]][t], 2,        RDsolR\[Rho]a[Rast, \[Rho], a0*au][[2]][t], 3, RDsolR\[Rho]a[Rast, \[Rho], a0*au][[1]][t]*(1 - RDsolR\[Rho]a[Rast, \[Rho], a0*au][[2]][t])],      scale[sel_] := Switch[sel, 1 | 3, au, 2, 1]}]

I have tried to use Epilog but no line was displayed.

Any help would be appreciated.