Traffic graph/widget - Remember user choice of showing bits/bytes between uses.  Currently lost and must be re-clicked every time traffic graph is viewed.

Notes:
1) "bandwidth_by_ip" is not used elsewhere, so adding "bits/bytes" to output appears safe.
2) Unclear if "null_call()" in "graph.php" is correct Ajax arg if no response will be needed; works though.
3) Unclear to me whether or not 2 files in "pfsense_packages/config/rate" need matching changes (See "bandwidth_by_ip" and "status_graph.php")