How to make smooth curve

Hi I have been trying for awhile to make a smooth curve instate of linear curve for my written script, but I've not gotten any kind of positive result .. I used spline, and polyfit did not work perfectly with mine, the matlab keep given me linear curve!. Let me share with you my codes

clc, clear all

disp(['The simulation requires to fill out the value of requirments'... 
    'of concepts']);
d=5;
h=10;
y_i=7;
y_s=5;
dt=30;
band =1;
v_in =100;
v_out =60;

% convert gallon per minutes to ft^3/s & rate it to be step every dt
v_inn = (v_in /448.831)*dt; v_outt=(v_out/448.831)*dt;
% volume equation
volume = @(current_hight) (d/2)^2*pi* current_hight;
% low band & high band
band_low= y_s - (1/2 *band); band_high= y_s + (1/2 *band);

% Possibilities
if volume(y_i) > volume(h) || volume(y_i) < volume(y_s)
    error('The amount of fluid is exceed the size of tank')
end
if v_out > v_in
    error(['The amount of outlet fluid is greater than inlet fluid,'...
        ' will not able to fill the tank'])
end

current_volume = volume(y_i); n= 2; 
out(1,1)=current_volume; 

for i = volume(y_i): - v_outt : (volume(band_low))
    current_volume = current_volume - v_outt;
    out(n,1)=current_volume; n=n+1;
    if current_volume <= volume(band_low)
        while current_volume < volume(band_high)
            current_volume = current_volume + v_inn - v_outt;
            out(n,1)=current_volume; n= n+1;
        end
    end
end
time = [0: dt : (length(out)-1)*dt];
water_level = out/((d/2)^2*pi);
hold on; box on; xlabel('Time(s)'); ylabel('Water Level (ft)');
title('On/Off Control Simulation');
axis([time(1) time(end) band_low*0.75 band_high*1.5]);
plot(time, water_level, '-ok','LineWidth',1);
plot(time, ones(length(out),1).*band_high,'-b','LineWidth',4);
plot(time, ones(length(out),1).*y_s, '-y','LineWidth',2);
plot(time, ones(length(out),1).*band_low,'-r','LineWidth',4);
legend ('Fluid Path Simulation', 'Upper Band','Setpoint','Lower Band')  
hold off

I would from

plot(time, water_level, '-ok','LineWidth',1);

to be a smooth curve line instate of linearly plot.

Thank you

[Jan]

I recommend to post the relevant code only. It is not clear what you mean by "smooth". The line has a sawtooth shape, so how coulöd it be "smoothed"? What did you try by "spline" or "polyfit"?