Subtract Static Value From Every Aggregate Value

InfluxDB 2
,
1

I have a time series set of data shown below and the returned values of the first occurrence of rx and tx values.

I want to subtract r_start from every returned value of rx_mean. This would result in a line that starts at 0 instead of whatever value is selected for the time span.

How would I subtract a static value from a aggregate series?

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SELECT mean_rx, mean_tx, r_start, t_start FROM 
  (SELECT first(tx_bytes) as t_start FROM usg_wan_ports WHERE $timeFilter),
  (SELECT last(tx_bytes) as t_end FROM usg_wan_ports WHERE $timeFilter),
  (SELECT first(rx_bytes) as r_start FROM usg_wan_ports WHERE $timeFilter),
  (SELECT last(rx_bytes) as r_end FROM usg_wan_ports WHERE $timeFilter),
  (SELECT mean("rx_bytes") as mean_rx FROM usg_wan_ports WHERE $timeFilter GROUP BY time($__interval) fill(null)),
  (SELECT mean("tx_bytes") as mean_tx FROM usg_wan_ports WHERE $timeFilter GROUP BY time($__interval) fill(null))