15.11.11  X...  

This  specifies which axes to bin.  X is always binned.  If Y is specified
then both X and Y are binned.   After  X  or  Y  you  should  specify  the
FROM,TO,BY options if you wish to bin into a specific range, or step size.
If you specify Y without MESH then a set of data sets are  produced,  each
for a different Y.