Hi Lin
I'm attaching a code below which maybe what you are looking for. The plot returns the figure-1 with six signals that you have provided, but with only a single vertical line to indicate the frequency as required.
data1 = importdata('short_0washer.mat','data');
data2 = importdata('short_2washer.mat','data');
data3 = importdata('short_4washer.mat','data');
data4 = importdata('short_6washer.mat','data');
data5 = importdata('short_8washer.mat','data');
data6 = importdata('short_10washer.mat','data');
t1 = data1(:,1);
t2 = data2(:,1);
t3 = data3(:,1);
t4 = data4(:,1);
t5 = data5(:,1);
t6 = data6(:,1);
Amp1 = data1(:,2);
Amp2 = data2(:,2);
Amp3 = data3(:,2);
Amp4 = data4(:,2);
Amp5 = data5(:,2);
Amp6 = data6(:,2);
Fs = 1/(t1(2) - t1(1)); % Sampling frequency
L = length(data1(:,1)); % Length of signal
f = Fs*(0:(L/2))/L;
Y1 = abs(fft(Amp1-mean(Amp1)));
Y2 = abs(fft(Amp2-mean(Amp2)));
Y3 = abs(fft(Amp3-mean(Amp3)));
Y4 = abs(fft(Amp4-mean(Amp4)));
Y5 = abs(fft(Amp5-mean(Amp5)));
Y6 = abs(fft(Amp6-mean(Amp6)));
P1 = 2*abs(Y1(1:length(f)))/L;
P2 = 2*abs(Y2(1:length(f)))/L;
P3 = 2*abs(Y3(1:length(f)))/L;
P4 = 2*abs(Y4(1:length(f)))/L;
P5 = 2*abs(Y5(1:length(f)))/L;
P6 = 2*abs(Y6(1:length(f)))/L;
hold on
ymin=0;
ymax=1;
y=[ymin,ymax];
[val, idx] = max(P1);
x=[idx/10,idx/10];
plot(x,y)
[val, idx] = max(P2);
x=[idx/10,idx/10];
plot(x,y)
[val, idx] = max(P3);
x=[idx/10,idx/10];
plot(x,y)
[val, idx] = max(P4);
x=[idx/10,idx/10];
plot(x,y)
[val, idx] = max(P5);
x=[idx/10,idx/10];
plot(x,y)
[val, idx] = max(P6);
x=[idx/10,idx/10];
plot(x,y)
Hope this answers your query. Cheers
