Since you need to plot "plot the Delta T (which is the difference) vs the time vector column" you need to have T be first and DeltaT be second in the plot() function:
% T = sort(rand(1, 100)); % Whatever....
DeltaT = diff(T);
% Plot the function.
plot(T(1:end-1), DeltaT, 'bo-', 'LineWidth', 2, 'MarkerSize', 8);
grid on;
xlabel('T', 'FontSize', 15);
ylabel('Delta T', 'FontSize', 15);
title('Delta T vs. T', 'FontSize', 15);
