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
.