Extracting and plotting max/min values from timeseries

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Lindazeta 2020-5-12

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https://ww2.mathworks.cn/matlabcentral/answers/524939-extracting-and-plotting-max-min-values-from-timeseries

评论： Lindazeta 2020-5-13

data.csv

I have a time series with continuous measurements of pH. The measurements are every 5 minutes, and the timeseries is a total of 13 days.

I want to find and plot the max and min value for each day, where the max and min values are based on 24 observations of a mean pH/hour.

So far I've been able to get the mean pH per hour, but I don't know how to find the max/min values from these and then plot as a continuous time series.

This is what I've done so far to find the mean pH for every hour:

dnv = [time];
dv6 = datevec(dnv);                                     % Date vector matrix, 6 columns
dn4 = datenum([dv6(:,1:4) repmat([0 0],size(dv6,1),1)]);
[Du,~,di] = unique(dn4, 'stable');
Means = accumarray(di, pH, [], @mean);              % Creating means for every hour
Out = [datevec(Du)  Means];                         % Output separated columns, date time mean
de = datetime(Out(:,1:6));                          % Merged date and time back into 1 column

So, how do I find the max and min values with the associated date and time for plotting?

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Answer 1

Peng Li 2020-5-12

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https://ww2.mathworks.cn/matlabcentral/answers/524939-extracting-and-plotting-max-min-values-from-timeseries#answer_432067

Better take advantage of table format and splitapply function.

tbl = readtable('data.csv');
tbl.day  = day(tbl.Date);
tbl.hour = hour(tbl.Date);
 
[Group, hourTbl] = findgroups(tbl(:, {'day', 'hour'}));
hourTbl.mean = splitapply(@nanmean, tbl.pH, Group);
[Group, dayTbl]  = findgroups(hourTbl(:, 'day'));
dayTbl.max = splitapply(@max, hourTbl.mean, Group);
dayTbl.min = splitapply(@min, hourTbl.mean, Group);