To find the equation for the SD line, use two known facts:
1) The SD line has a slope b=SDy/SDx=1.025441
2) The SD line passes through the center of regression, (mean(X), mean(Y))=(67.6871, 68.68407)
So the equation is given as:
y-68.68407=(x-67.6871)*b,
which by re-arranging yields:
y=1.025441*x + (68.68407-67.6871*1.025441).
From this you can find the value of a (intercept) and b (slope) of the SD line.
To add the SD line, use R command
abline(a=a,b=b,col="blue", lty=4, lwd=3);