stem command working on the oppisite side of the axis

I want to create a plot of low pass filter ,while the Y axis is in dB and x is the ferquincy in logaritmich scale. I made the plot and now I want using the stem command to point to the cufoff ferquincy . but for some reason when I want to point to the cut off ferquincy the stem comand making the line from the up and not from down . how to fix this problem ?

clc
close all;

y=mag2db(dB_axes(1:120));
x=f_axes(1:120);

for i=1:length(mag2db(y(1:120)))
    
    if(2.8<abs(y(i)) && abs(y(i))<3.5 )
        i; %% find the cutoff ferquincy 
        break;
    end;
    
end
plot(x,y,'red','linewidth',3);
grid on;
set(gca, 'XScale', 'log');
hold on
stem(x(i),y(i),'*','linewidth',2);
xlabel('f[logarthmic scale]');
ylabel('dB');
title('RC filter -freuquincy responce');

Answer 1

[Star Strider]

It is necessary to give stem a different base value in order for it to plot upwards to a negative value.

Try this —

s = tf('s');
sys = 1/(s^2+2*s-5);

[a,p,w] = bode(sys);
bw = bandwidth(sys);
asq = squeeze(a);
hppa = interp1(w, mag2db(asq), bw);

figure
semilogx(w, mag2db(asq))
hold on
stem(bw, hppa, 'BaseValue',min(mag2db(asq)))
hold off
grid

.

Answer 2

[Clayton Gotberg]

This is because you haven't provided any information about the baseline for stem, so the function assumes you want it to come from y = 0. To adjust it to come from the bottom of the chart, use this:

ax = gca;
y_min = min(ax.Ylim)
stem(x(i),y(i),'*','linewidth',2,'BaseValue',y_min);