About differentiation of plot after filtering the vector using filter(b,a,x)

I have a problem that I want to difrentiate the data to find the maximum but my data contains small spikes....so i need to filter it first..

with command filter it is not working because it takes approximation of past values so the vector elements becomes same,,,, making diffrentiation zero!!!

please find the attachment and

[rathod ujjval rameshbhai]

clc;clear; open('C:\Users\UJJVAL\Desktop\model_input1_Ziegler_Nicholus.fig') h=gcf; get(h); ax=gca; T=get(ax,'children'); t=get(T,'XData'); y=get(T,'YData'); a = 1; b = [1/4 1/4]; f=filter(b,a,y); plot(t(7:end),f(7:end)); df=(diff(f,1,2)./diff(t,1,2)); ylim([0 0.5]);plot(t(2:end),df);grid on;