How to find the peak value of a graph and how to plot them?

I need to find the peak value of each force for the plotted graph and locate it in the graph. I already use findpeaks but there is no answer and no error informed. I'm not sure the flow and how to apply the findpeaks command. X=xlsread('Filtered JCF PHASE 1.xlsx',4,'Z5:AC55'); Y=xlsread('JCF PHASE 1_5UP.xlsx',4,'AN5:AP55'); Z=xlsread('JCF PHASE 1_5DW.xlsx',4,'AN5:AP55'); x1 = X(:,1); %the percentage y1= X(:,2); %the force value 1 y2= Y(:,1); %the force value 2 y3= Z(:,1); %the force value 3 [pks,locs] = findpeaks (y1,x1); findpeaks (y1, x1); subplot(3,1,1); plot(x1,y1,'r',x1,y2,'g',x1,y3,'b','Linewidth',3); title('JCF HIP X'); xlhand = get(gca,'xlabel'); set(xlhand,'string','Percentage of stance phase (%)','fontsize',10) ylhand = get(gca,'ylabel'); set(ylhand,'string',{'JCF (N)'},'fontsize',10);