Graph Panel - display first value and last value in timeseries (connected by line)

I’ve searched and perhaps I’m not using the proper search terms. I’m trying to build a single metric that selects the first value of a field based on time_filter and the last value of the same field based on time_filter.

I’d like to display a straight trend line with only the first value and the last value in the time series. Can anyone help me build a query that will do this?

I’ve tried:
SELECT first(value) AS firstval,
last(value) AS lastval
FROM whatever
WHERE $timeFilter

but it’s displaying both points on the same X-Axis. I think I may need a GROUP BY clause in there but I’m stuck.

Does anyone know how I can accomplish this?