The proximate error is that the symbols around the year-range string
‘1931-1960’
are not apostrophes, so MATLAB doesn't know what to do with them. They should appear in pink:
'1931-1960'
But that is a moot point, because you don't need to use the "g" argument in boxplot for this problem. The g argument is used to group heterogenous elements of each column in x together in seperate boxplots. You're data is already homogenous in each vector, so you can just pass three columns in x to get three boxplots. The trick is to handle their unequal lengths, which you can do by filling a just-large-enough NaN matrix x. The extra elements in each column are ignored by boxplot.
N = max( [ numel(DailyTMAX_1), numel(DailyTMAX_2), numel(DailyTMAX_3) ] ); % Longest length
x = NaN(N,3); % 3 columns large enough to fit longest vector
x(1:numel(DailyTMAX_1),1) = DailyTMAX_1;
x(1:numel(DailyTMAX_2),2) = DailyTMAX_2;
x(1:numel(DailyTMAX_3),3) = DailyTMAX_3;
boxplot(x);
You can greatly simplify this by storing "DailyTMAX_X" vectors natively in a NaN-padded x matrix.
Note: this won't add the year-labels, you can do that manually by:
set( gca, 'XTickLabel', {'1931-1960', '1961-1990', '1991-2020'} );
