Obtain RR-Interval and plot it over time

Dear Matlab Community,

My function RRInterval(...) is supposed to output an array of the RR values, as well as the RR_location of them. Once plotted in milliseconds over time, I would like to extract the breathing rate, as the following image represents:

https://www.researchgate.net/figure/Illustration-how-to-derive-the-tachogram-RR-interval-time-series-from-the-ECG-signal_fig9_267403305

I created a function because I have 4 different ECGs, 3 capacitive ECGs + 1 reference ECG. They represent different leads on how the electrodes are positioned on a chair (see image below).

So far so good, but now I struggled on the Matlab-code, in which I need your help :)

function [RR,RR_loc] = RRInterval(ECG_Signal,record)
    
% Find peaks in all available capacitive ECG signals
peaks = findPeaksInRecord(record);

% Plot signals with the detected peaks
plotRecord(record,peaks) %first image

% Sampling frequency of 200 Hz
fs = 200;

% RR_location
% typeCode = 1 if ECG Signal detected, != 1 if an abnormality is detected
% amplitude x1000
[Rpeaks, typeCode] = findRPeaks(ECG_Signal, 1000, fs);

% Difference of Rpeaks
RR = diff(Rpeaks);
RR_loc = Rpeaks(1:end-1) + 0.5 * RR;
RR     = RR ./ fs;
RR_loc = RR_loc ./ fs;

end

With the use of the console, I plotted the RR from the cECG_2 over time, which it does not make sense to me (for the other cECG the figures look similar to this as well, sure):

So my questions are:

1. Am I approaching the extraction of the breathing rate properly? I would like to both measure it (it should be around 15-18 BPM) and then plotted it along with the ECG.
2. Why does my RRInterval differs from the theory-image? What am I missing?
3. Does it make sense to extract the RRInterval for each cECG? And therefore get 3 different breathing rate signals? Or how can I previoulsy mixed them together?

Thank you for your time,

David