Try it this way:
% t=[0:0.0002:20];
% xc1=cos(2*pi*493.88*t);
% argument in cos = 2*pi*t/period. So
period = 1/493.88
threePeriods = 3 * period
% Make 1000 points over that 3 period.
t = linspace(0, threePeriods, 1000);
% Make xc1 with that t:
xc1 = cos(2 * pi * t / period);
% Plot it
plot(t, xc1, 'b-', 'LineWidth', 2);
xlabel('t', 'FontSize', 15);
ylabel('xcl', 'FontSize', 15);
grid on;
caption = sprintf('Three Periods of cos(2 * pi * t / period), where period = %f', period);
title(caption, 'FontSize', 15);
