Problems with plotting a seperation line before pdeplot

Hello guys,

currently I am working on a programme which is able to show the heat transfer on two layers illustrating different industrial materials. In order to visualize each layer i would like to add a seperation line where one layer starts and the other begins. It works fine when i use the following code:

pdeplot(thermalmodel,'XYData',T,'Contour','on','ColorMap','hot')
hold on
y = linspace(0,3,10)
plot(3,y,'--bo')

However, I would like to first plot the seperation line and then add the data from the heat transfer. I've tried to just change the order:

y = linspace(0,3,10)
plot(3,y,'--bo')
hold on
pdeplot(thermalmodel,'XYData',T,'Contour','on','ColorMap','hot')

The heat transfer plot still works fine but the seperation line is missing here. Does anyone know why that is the case and what can be done in order the get both plots while keeping this order?