You pretty much have it laid out:
diff_data=diff(data);
diff_data=[0;diff_data];
percentageChange = abs(diff_data)./abs(data);
index = find(percentageChange>.5);
plot(data)
hold on
plot(index, data(index), '*')
The code above can help you detect change greater than 50% comparing to the previous point. If you want to compare 3 to 5 seconds you need to modify the diff part:
compareToPreviousData = 3; % Then you can compare to data 3 seconds ago
index = detectChange(data, compareToPreviousData);
plot(data)
hold on
plot(index, data(index), '*')
function index = detectChange(data, numSec)
diff_data = data(numSec+1:end) - data(1:end-numSec);
size(diff_data)
diff_data=[zeros(numSec, 1);diff_data];
percentageChange = abs(diff_data)./abs(data);
index = find(percentageChange>.5);
end
