Apparently, the first set call was the problem. I don’t have your ‘matrix’ variable, but for datetick to work, matrix(:,1) have to be date numbers. After that, combine the set arguments in one call.
This works:
matrix(:,1) = linspace(733645,736117,20);
matrix(:,7) = randn(1,20);
plot(matrix(:,1),matrix(:,7));
axis tight
xlabel('Date');
ylabel('PnL');
datetick('x',2,'keepticks'); %convert serial number to date
set(gca,'xtick',linspace(733645,736117,50), 'XTickLabelRotation',45, 'FontSize',6); % label 10 ticks using start/end
