Polyfit just returns the coefficient values. If you want to plot it, you need to evaluate your polynomial, and then plot the resulting values using plot.
x = linspace(0,4*pi,10);
y = sin(x);
p = polyfit(x,y,7);
% evaluate polynomial
x1 = linspace(0,4*pi);
y1 = polyval(p,x1);
% plot
scatter(x,y)
hold on
plot(x1,y1)
hold off
legend('Data','Polyfit')
