I'm using the bubble chart with a constant bubble size to mimic a scatter plot. I was having trouble getting my plots to make sense, so I threw in X/Y pair where X == Y (though the data was unsorted), and the data points were all over the place. I then tried sorting the data ascending, and voila, I get my expected straight line. Curiously, if I sort the data descending, I get a line with just one or two of the points misplaced.
If you need me to post code to reproduce this I'll be glad to, but I'm guessing you can reproduce it easily. The data points below yield the problem for me.