How do I change a ColorFunction filling to be top-down as opposed to left-right? [duplicate]

Question

I've been stuck on a seemingly simple Graphics issue for quite a while now...

I am trying to make a temperature distribution Plot that will have a colorFunction filling to show red when at a higher temperature, then fading to blue as it gets colder, etc. My plot has temperature in the y-axis, and length (of a fin) in the x-axis.

When I try to put it into effect, the filling is filled with the gradient but it goes from left to right, along the x-axis. However, I want it to be fading top to bottom, to illustrate how the temperature decreases. One of my cells of test code is included below:

    Manipulate[
 Plot[T[x, length, h, P, A, k, Tinf, T0], {x, 0, length},
  ColorFunction -> 
   Function[{x, T}, 
    Directive[
     Opacity[0.7], (ColorData["DeepSeaColors"][Rescale[T, {0, 50}]])]],
  ColorFunctionScaling -> False,
  ImageSize -> 200,
  PlotRange -> {0, T0}, 
  Filling -> Axis,
  PlotStyle -> Thick,
   AxesLabel -> {x, \[Theta]}],

 {{length, 5}, 1, 30, 1, Appearance -> "Labeled"}
 ]

I've tried a lot of convoluted methods like making a customized Raster image for the filling color, but I'm sure there has to be a simpler way to do this!

EDIT:

I want to be able to do this with the Plot function, so it is easier to Manipulate for the user. Right now I was able to get it to work by creating the data separately and then using a ListLinePlot, like so:

heatData = Transpose[{Range[100], Table[-0.5 x + 50, {x, 1, 100}]}];
bands = 50;

ListLinePlot[Table[{1, i} # & /@ heatData, {i, 0`, 1`, 1/bands}], 
 Background -> White, ColorFunction -> "TemperatureMap", 
Filling -> True]

Thank you so much!