MATLAB: Error when fitting polyfit curve

errorpolyfittimeseries

Trying to plot a polynomial curve onto my time series but keeps coming up with 'Inputs must be floats, namely single or double'
This is the code I've got:
% plot time series
X = Months;
X = categorical({'March','April','May','June','July','August','November'});
M = reordercats(X,{'March','April','May','June','July','August','November'});
Y = AverageDO;
plot(M,Y,'*');
p = polyfit(M,Y,1);
The excel sheet attached is the timeseries data.

Best Answer

Error is exactly what it says--you wrote:
X = categorical({'March','April','May','June','July','August','November'});
M = reordercats(X,{'March','April','May','June','July','August','November'});
so M is a categorical variable which polyfit is not equipped to handle.
Use
>> M=month(datetime(string(X),'inputformat','MMM'))
M =
     3     4     5     6     7     8    11
>>
or some other way to generate the months as numeric.
I THINK some of the Statistics and/or Curve Fitting toolbox routines may know how to handle categorical variables but I don't recall for absolute sure otomh.
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