I'm trying to convert square wave data to instantaneous frequency and plot against time. It works correctly for one data set, but three others are plotting as shown in the image: http://i.imgur.com/iuL648o.png Time is linear, so it shouldn't be plotting like that. Anyone have any ideas as to what's going on? I'm using the following code: ddiff = diff(flow(:,1)); locations = find(ddiff(1:end-1,1)>0.1 & ddiff(2:end,1) <=0.1); for z = 2:length(locations) inst_freq(z-1) = 1/(time(locations(z))-time(locations(z-1))); freq_time(z-1) = time(locations(z)); end plot(freq_time,inst_freq)