An
internet search brought me to
a page that indicates that xgraph is only available on Xenial, i.e. Ubuntu 16 (the search results also included references to Trusty [Ubuntu 14], but I believe it is not supported anymore).
If you run Xenial, you need to find out why this package is not found. Check the apt sources configured on your system.
Alternatively, or if you don't run Xenial, install it from the
xgraph web site.
