Colored gradient fill under curve

I know this is not the most elegant solution, but it works. Indeed, I am a beginner, so excuse me. My question is, how to simplify it all?

Here's what I want to achieve: I have a 2D plot and I want to make a gradient fill under curve. From one specific color to transparent. I tried to use white instead of transparent, but I want the grid to be visible. So I'm little lost here.

Did some search and I looked for some examples here and tried to modify them. So, this is an edited code that worked to fill a specific range. I spread it all over the axis. I also didn't want to fill the gradient over the edge of the curve, so I had to insert plot again. Yeah, that's not ideal at all, but as I said, it's working.

I believe that this can be done more easily. Could you advise me, please?

% Input data
x = 0:0.01:10*pi;           
y1 = sin(x)+9; 

y2 = repmat(5,length(y1),1);
figure1 = figure;
plot(x,y1);
grid on;
hold on;

% Fill the whole area - 0-31.4
val = [0,31.4];
for i = 1:2;
    tmp = abs(x-val(i));
    [~,idx(i)] = min(tmp);
end
id = idx(1):1:idx(2);

x2 = x(id);
y1a = y1(id);
y2a = y2(id);
y2a = y2a';

X=[x2,fliplr(x2)];
Y=[y1a,fliplr(y2a)];

% I set the transparency and removed borders. 
% This is a workaround, I don't want to set the whole transparency.
h = fill(X,Y,Y,'LineStyle','none');
set(h,'facealpha',.5)

% Here is how I made the gradient between red and white.
% This is not very nice solution and I would like to simplify it. 
% Of course, color bar is not visible.
set(gca,'Colormap',[1 1 1; '...a lot of data...' ;1 0 0]);

% Need to plot again because of overlaping
plot(x,y1);

Answer 1

[darova]

Use alphaData property, set facealpha to interp

x = 0:0.2:10;
y = sin(x)+10;
xx = [x;x];
yy = [y;y*0];
surf(xx,yy,xx*0,...
    'alphadata',yy,...
    'facealpha','interp',...
    'edgecolor','none');
line([0 x x(end) 0],[0 y 0 0],'linew',2)
view(2)

[Leo]

I have also a related question: Is there any option how to set a different Alim for multiple surf plots? For example I have surf1 with height 20, another surf2 with height 10. Unfortunately gradient for both starts from the highest number 20 so I'm unable set it for each separately.

x = 0:0.1:50;
y = 5*sin(x)+5;
y2 = 5*sin(x+5)+15;
xx = [x;x];
yy = [y;y*0];
yy2 = [y2;y2*0];
Color1 = [0.72 0.28 1];
Color2 = [0 0.45 0.74];

figure

alim([0 max(y)]); % NOT WORKING!
s1=surf(xx,yy,xx*0,...
    'alphadata',yy,...
    'facealpha','interp',...
    'edgecolor','none',...
    "FaceColor",Color1);

hold on

alim([0 max(y2)]); % NOT WORKING!
s2=surf(xx,yy2,xx*0,...
    'alphadata',yy2,...
    'facealpha','interp',...
    'edgecolor','none',...
    "FaceColor",Color2);

view(2)