Survey summary image can be mysteriously broken

See MDLSITE-3772. If student groups or capabilities are misconfigured the student can get to the page where the summary image should be but the image itself does not display. No explanation is given for why.

The image has a URL like https://learn.moodle.net/mod/survey/graph.php?id=28074&sid=21409&group=0&type=student.png Opening that URL in a new tab takes you to a page that says "No permission to see this!"

This appears to be due to either a problem with capabilities or groups. The page containing the image should really give some sort of clue why the image is not visible. This seems to be the relevant check in graph.php

if (!has_capability('mod/survey:readresponses', $context)) {

        if ($type != "student.png" or $sid != $USER->id ) {

            print_error('nopermissiontoshow');

        } else if ($groupmode and !groups_is_member($group)) {

            print_error('nopermissiontoshow');

        }

    }