# Customizing Streamplot with other plots

I am trying plot vector field of a dynamical system together with highlighting some area using some inequality constraints.

My state-variable for vector fields are $$x$$ and $$y$$.

The following are the parameters used for the vector field

ls = 2*10^-3;
cs =  2*10^-3;
g = 0.03;
p1 = 1000;
pi1 = 1100;
is = 0;
ki = 0; kd = 1; yd = 380;


The vector fields are given by f1 and f2:

f1 = -(pi1/y^2 + kd)* cs^-1*(x - g*y - p1/y - is) - (ki + ls^-1)*(y - yd);
f2 = cs^-1*(x - g*y - p1/y - is);


I have used the following code to Draw the vector SteamDensity plot

StreamDensityPlot[
{f1, f2}, {x, xd - 100, xd + 100}, {y, yd - 100, yd + 100},
StreamPoints -> {{{{xd, yd}, Blue}, {{xd - 50, yd - 50},
Green}, {{xd + 50, yd + 50}, Red}, Automatic}},
PlotRange -> {{xd - 100, xd + 100}, {yd - 100, yd + 100}},
FrameTicks -> {{-50, {14,
"\!$$\*SuperscriptBox[\(I$$, $$*$$]\)=14 A", {.5, 0},
Thickness[0.001]}, 50}, {250, 300,
350, {380, "\!$$\*SuperscriptBox[\(V$$, $$*$$]\)=380 V", {.5, 0},
Thickness[0.001]}, 450}},
AspectRatio -> 1,
FrameLabel -> {Current  , Voltage },
PlotLegends -> Automatic,
PlotTheme -> "Monochrome",
StreamPoints -> Fine,
TicksStyle -> Directive[Orange, 50]
]


This resulted in the following plot: However, I am want to highlight the area inside the plot satisfying $$370. I checked all the option of StreamDensityPlot but didn't find any relevant function that does this.

Try putting a Rectangle in Epilog:

StreamDensityPlot[{f1, f2}, {x, xd - 100, xd + 100}, {y, yd - 100,
yd + 100},
StreamPoints -> {{{{xd, yd}, Blue}, {{xd - 50, yd - 50},
Green}, {{xd + 50, yd + 50}, Red}, Automatic}},
PlotRange -> {{xd - 100, xd + 100}, {yd - 100, yd + 100}},
FrameTicks -> {{-50, {14,
"\!$$\*SuperscriptBox[\(I$$, $$*$$]\)=14 A", {.5, 0},
Thickness[0.001]}, 50}, {250, 300,
350, {380, "\!$$\*SuperscriptBox[\(V$$, $$*$$]\)=380 V", {.5, 0},
Thickness[0.001]}, 450}}, AspectRatio -> 1,
FrameLabel -> {Current, Voltage}, PlotLegends -> Automatic,
PlotTheme -> "Monochrome", StreamPoints -> Fine,
TicksStyle -> Directive[Orange, 50],
Epilog -> {Orange, Opacity[0.5],
Rectangle[{xd - 100, 370}, {xd + 100, 410}]}] 