Set the legend text. Legends can contain variables based on the
exact keys matched in the query clause and keys used in a
group by. Variables start with a $ sign and can optionally
be enclosed between parentheses. The parentheses are required for cases
where the characters immediately following the name could be a part
of the name. If a variable is not defined, then the name of the variable
will be used as the substitution value.
The variable atlas.offset can be used to indicate the time shift
used for the underlying data.