There is nothing obvious that you're doing wrong, it should work in this way. (I suppose you have a similar mapping for y.)
Can you please check that importing the data from Excel worked correctly: select a node and check that it's Data values in the Properties View dock panel are correct.
If there is nothing wrong with these data values, please upload your GraphML file to your post (if it doesn't contain any confidential data): http://yed.yworks.com/support/qa/1734/
I'll then have a closer look at the problem.