Try it this way, where you pass only the handles of some of the plots into legend(). Plus you don't need to call hold on over and over again. Once it's called, it's set until you call hold off or clear the axes.
%=======================================================
% Create sample data
numPoints = 10;
t_1 = 1:numPoints;
CH0_1 = rand(1, numPoints);
t_2 = 1:numPoints;
CH0_2 = rand(1, numPoints);
t_7 = 1:numPoints;
CH0_7 = rand(1, numPoints);
t_8 = 1:numPoints;
CH0_8 = rand(1, numPoints);
t_10 = 1:numPoints;
CH0_10 = rand(1, numPoints);
t_11 = 1:numPoints;
CH0_11 = rand(1, numPoints);
%=======================================================
% Now plot the data.
% Plot T1 and T2 combined together
h1 = plot(t_1,CH0_1,'r', 'LineWidth', 3);
fontSize = 16;
title('Measurement of CHO vs. t', 'FontSize', fontSize);
xlabel('t', 'FontSize', fontSize);
ylabel('CHO', 'FontSize', fontSize);
hold on;
plot(t_2,CH0_2,'r', 'LineWidth', 3);
% Plot T7 and T8 combined together
h2 = plot(t_7,CH0_7,'m', 'LineWidth', 3);
plot(t_8,CH0_8,'m', 'LineWidth', 3);
% Plot T10 and T11 combined together
h3 = plot(t_10,CH0_10,'b', 'LineWidth', 3);
plot(t_11,CH0_11,'b', 'LineWidth', 3);
% Show legend
legend([h1,h2,h3], 'Data1','Data2','Data3'); % Legend for all plots
grid on;
