I apologize if what I responded with did not answer your question. Given the code you provided, I assumed that you were creating an individual chart and that the issue was just an issue of the X axis being too cluttered and because of that, you only wanted to display the day as the label, because all of your data for the chart were for just one day.
I added the subgroup options, because it gave an easy way to change the displayed label.
Apparently I did not interpret your statements correctly
Here is another approach that does not use a subgroup
theLabel = format(:testtime[1],"m/d/y");
ccb = Control Chart Builder(
Variables( Y( :Y ) ),
Show Control Panel( 0 )
);
report(ccb)["Control Chart Builder",TextEditBox(4)] << set text( theLabel );
.
Jim