Since you haven't said what you actually attempted and haven't given any sample data or even revealed the domain and range of the plot image, I can only offer a simple example.
y = -0.01*(x-6).^2 + 0.05*abs(sin(4*x));
fm = fit(x',y','smoothingspline','smoothingparam',0.9);
plot(x,y,'k:'); hold on; grid on
You'd obviously have to tailor the parameters to whatever data you're actually using.