that Scatter Graphs only compares two sets of values in X and Y. See the examples here;
http://www.comfsm.fm/~dleeling/statistics/xyscattergraph.html

That means you can select any two of the data sets you have eg salary and pay grade, or salary and years of service, or pay grade and years of service.

In your example you have the Y axis as salary and the X axis as pay grade and years. The X axis can only plot one value, not two.

You will need to find some other solution to this, perhaps have 3 scatter graphs, one each for the 3 pay grades, and each comparing Salary against years of service 'within' the respective pay grades.

Sorry. In fact thinking about it, using Pay Grades as the title for the graphs may be more acceptable to those who will see them, and make more sense.

Mark