This function draws the "bias plot", which is used to visually assess the
bias of the new method relative to the reference method. It is obtained by
graphing a scatter plot of y1 (new method) and y2 (reference method) versus
the BLUP of the latent trait, x, along with the two regression lines.
The function adds a second scale on the right axis, showing the relationship
between the estimated amount of bias and BLUP of the latent trait, x.