Instead of using datenum x values, you can plot with datetime x values and then set the xticklabel format using xtickformat:
% some datenum values:
x = [7.35e5 7.36e5 7.37e5];
% convert datenum to datetime (it's likely you can bypass this conversion
% and use datetime() on your data directly)
x = datetime(x,'ConvertFrom','datenum')
% plot with datetimes:
plot(x,[3 2 4])
% set the xtick format:
% xtickformat('yyyy-MM') % take your pick
xtickformat('yyyy')
