In certificate 2013102300, certificate_get_grade() returns the a string with text indicating where the grade is from (e.g. the course, of the title of a quiz), followed by the grade as points, percentage, or letter.

For custom certificate types, we sometimes need to get the grade without the text.

I'm working with certificate 2013102300 on Moodle master, but would expect it to apply on other Moodle versions.