Use a text (link) object. Choose the (x,y) coordinates where you want it to appear, then after you plot your data:
text(x, y, sprintf('R = %.2f', R))
Experiment to get the result you want.
EDIT — To add your ‘R’ value to a legend entry:
R_1 = 0.42;
R_2 = 0.21;
figure(1)
plot(rand(1,20), rand(1,20), 'pg', rand(1,20), rand(1,20), 'pr')
legend(sprintf('Data_1 (R = %.2f)',R_1), sprintf('Data_2 (R = %.2f)',R_2))
