The various labels are all accessible via the Tree Structure of the Output Report. To learn about the them, read the sections in the Scripting Guide on Display Trees. The Scripting Guide can be found in the JMP Documentation Library.
Help==>JMP Documentation Library
Here is a little script that changes the various Labels on this style of graph
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
gb = Graph Builder(
Variables( X( :age ), Y( :height ), Group Y( :sex ) ),
Elements( Bar( X, Y, Legend( 1 ) ) )
);
gbr = gb << report;
gbr[Text Edit Box( 1 )] << set text( " This is the Header" );
gbr[Text Edit Box( 4 )] << set text( " This is the Y Axis Label" );
gbr[Text Edit Box( 3 )] << set text( " This is the X Axis Label" );
Jim