Hi:
I suggest that you search support.sas.com on the search argument:
ALT= HTML tool tips graph
A few hits that come up are these:
http://support.sas.com/kb/23/176.html
http://support.sas.com/kb/23/199.html
Which shows how to code ALT= using the HTML= option. Generally, the HTML= option is pointing to a drilldown file, so ALT= is an acceptable attribute to add to the drilldown HREF. Even though these are not SHEWHART examples, it was my understanding that HTML= worked the same way in all the procedures that supported HTML=.
For more help using HTML= option with PROC SHEWHART, you might consider contacting SAS Tech Support.
cynthia