how to apply a uniform color gradient to a patch object

hi I'm trying to show a filled histogram of selected images. The fill" area under the curve will indicate the color assignment determined by a user-specified colormap. I'm using a patch object to create the filled area. The problem I have is that the shading of the patch is not uniform from left to right, i.e. I expected vertical slabs of color but instead the patch shows odd diagonal-like shading that I have no idea how to remove. I I'm not sure what I'm doing wrong. A code that reproduces this behavior is shown below:

x = log10(sort(randi([900 65535],1,256)));
y = log10(randi([1 150000],1,256));
X = [x(1) x x(end)];
Y = [0 y 0];
cmap = colormap(gray(258));
figure
hline = line(x,y,'Color','r','LineStyle','-','LineWidth',2);
hold on
patch('Faces',1:size(X,2),'Vertices',[X' Y'],'FaceVertexCData',cmap,'FaceColor','interp')
hold off

Please help! best Ramiro email: [e-mail]; [e-mail]