a = [15.5156,0.1995;
7.6003,0.2999;
9.4829,0.2592;
12.2185,0.2239;
23.4094,0.1811];

x = a(:, 1);
y = a(:, 2);
n = 2; % order of the fitted polynomial trendline
p = polyfit(x, y, n);
m = 1000; % number of trendline points (the larger the smoother)
xx = linspace(min(x), max(x), m);
yy = polyval(p, xx);

figure;
hold on;
scatter(a(:,1), a(:,2));
plot(xx, yy, 'r-');

print out.png