/usr/share/mercurial/templates/raw/graph.tmpl is in mercurial-common 4.5.3-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | {header}
# HG graph
# Node ID {node}
# Rows shown {rows}
{nodes%graphnode}
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