I'm not sure what you mean by "continuous trend from left to right". Can you post an image of what you're looking for? This was what I thought you meant.
x = 1:30;
y = rand(1,length(x));
sz = rand(1,length(x));
%get color gradient from https://www.mathworks.com/matlabcentral/fileexchange/25536-red-blue-colormap
m = length(sz);
if (mod(m,2) == 0)
% From [0 0 1] to [1 1 1], then [1 1 1] to [1 0 0];
m1 = m*0.5;
r = (0:m1-1)'/max(m1-1,1);
g = r;
r = [r; ones(m1,1)];
g = [g; flipud(g)];
b = flipud(r);
else
% From [0 0 1] to [1 1 1] to [1 0 0];
m1 = floor(m*0.5);
r = (0:m1-1)'/max(m1,1);
g = r;
r = [r; ones(m1+1,1)];
g = [g; 1; flipud(g)];
b = flipud(r);
end
c = [r g b];
t = tiledlayout(1,1);
nexttile
b = bubblechart(x,y,sz,c,'MarkerEdgeColor','k');
colormap(c)
colorbar
blgd = bubblelegend('Population');
lgd = legend('My data');
blgd.Layout.Tile = 'east';
lgd.Layout.Tile = 'east';
