The order is set by the point position.

So for the following data set of X,Y,Size data you should see a set of
concentric bubbles. Where the last point with a size of 1 is on top.
Change the last points size to 5 to cover all subsequent points.
X    Y    Size
1    1    5
1    1    4
1    1    3
1    1    2
1    1    1

If you have more than 1 series the 2nd series is plotted over the 1st.

And finally any series on the secondary axis will be plotted above those
on the primary axis.

Cheers
Andy