Statistical Glossary
The Tukey MeanDifference Plot:
The Tukey meandifference plot is a scatter graph produced not for (x,y) values themselves, but for modified coordinates (X,Y)

Such a plot is useful, for example, to analyze data with strong correlation between x and y  when the (x,y) dots on the plot are close to the diagonal x=y. In this case, the value of the transformed variable X is about the same as x and y; and the variable Y shows the difference between x and y.
The Tukey meandifference plot is meaningful for two similar variables  that is, when both x and y are of the same physical dimension and expressed in the same units  e.g mass in pounds (or kilograms, ...), length in foots (or meters, ...). Otherwise, it makes no sense to sum up or subtract values of the variables x and y.