Specify an URL (relative or absolute)
Use special variables:
${__y_label} name of the time series (alias or legend in the query editor).
${__bucket_range} refers to bucket range in the form of '&from=...&to=...'.
${__graph_range} refers to graph range in the form of '&from=...&to=...'.
${__value} refers to current bucket value or max value if multiple values mode is on.
${__value_N} refers to Nth value in current bucket. Value indexes are started from 0. For instance,
${__value_1} refers to second value.