A few issues:
- <acronym title="Service Side Includes">SSIs</acronym>
with "exec" enabled can be dangerous, especially if you
(asusming it's your machine) are not the only one with the
ability to create SSI documents on the web server.
- Does the user "nobody" (or whoever the web server runs as)
have permission to modify the "num.dat" file?
- In the "num.html" case, perhaps the issue is that the file
and graphics are cached by the web browser.
Another option might be to use a dynamically generated graphic from\
a <acronym>CGI</acronym> script (this is a common example for
people learning CGI).