readChannelID = 123456; 
WeightFieldID = 1; 
readAPIKey = 'ABCDE'; 

AnzTage = 16;
StartDate = datetime('today') - days(AnzTage)

%Alternatively Start at a specific date
%StartDate = 'June 5, 2019';

%Read data from StartDate until yesterday ('today' liefert gestern(??))
[data,time] = thingSpeakRead(readChannelID,'DateRange',[StartDate,datetime('today')]);

%Determine number of days in dataset
DaysInRange = ceil(days((max(time) - min(time))));

%Extract Weight
Weight = [data(:,WeightFieldID)];

i = 1;
while i <= AnzTage & i <= DaysInRange %Find max value of each day
    %Let's do some datetime magic ;-)
    BeginDay = day(min(time) + days(i-1)); 
    BeginMonth = month(min(time) + days(i-1)); 
    BeginYear = year(min(time) + days(i-1)); 
    EndDay = day(min(time) + days(i)); 
    EndMonth = month(min(time) + days(i)); 
    EndYear = year(min(time) + days(i)); 
    
    BeginDate = datetime(BeginYear,BeginMonth,BeginDay);
    EndDate = datetime(EndYear,EndMonth,EndDay);
  
    index = isbetween(time,BeginDate,EndDate); %find all measurements of this day
    Zeit(i) = min(time(index));
    
    %Now let's get the max weight value of each day (however this method is sensitive to outliers...)
    %Gewicht(i) = max(Weight(index));
    
    %It may be more reliable to get the mean value of each day
    %Gewicht(i) = mean(Weight(index));
    
    %Even more intelligent way to find the "real" weight:
    DayWeight = Weight(index);
    %Take the last 10 values of each days - according to my experience
    %it seems the most stable time for getting the weight values 
    %(this is later than 10 p.m. with a measurement interval of 15 minutes, may be adjusted):
    Gewicht(i) = mean(DayWeight(end-9:end));
    
    %Let's go to next day...
    i = i + 1;
end

%Differentiate the weight to get the day-to-day differences
barh(Zeit(2:AnzTage),diff(Gewicht),0.8) %Bar chart horizontal
%bar(Zeit(2:AnzTage),diff(Gewicht),0.8) %Bar chart

%Summarize the day-to-day differences to get the cumulated weight change
CumWeight = sum(diff(Gewicht));

%Now let's draw the plot
title('Tägliche Gewichtsveränderung');
xlabel(['Veränderung gesamt: ' num2str(CumWeight,2) ' kg'])
%ylabel('kg') %uncomment for Bar chart
grid on