If you plot only 118 points you can't get more from the plot. The results are interpolated and probably what you want is a continuous formula to model y as a function of x. If you have a polynomial data, take a look at the polyfit function from matlab. In case you really only want to have more refined values, you just need to interpolate the inbetween results yourself:
x = 1:118;
y = x.^2;
xq = linspace(x(1),x(end),490); % Here is the new points you want to generate
yq = interp1(x,y,xq,'pchip'); % Here is the actually interpolation
figure,plot(x,y,'*'),hold on,plot(xq,yq)
