Select maximum value over the past 24 hrs

As a Grafana newbie I want to display the maximum temperature of the measured past 24 hours in a gauge. For that I use this query:

SELECT MAX("tempachtertuin") FROM "lucht" WHERE $timeFilter GROUP BY time(24h) fill(null)

Unfortunately that is not going well, the query shows the current temperature and not the maximum over 24 hours. I am too inexperienced, can someone tell me which query to use?

What about the option in the right side (“Calculation”) set to Max? In my example shown below, 203 F is the maximum temperature over the last 30 minutes. If I changed the time dropdown to 3 hours, it would show the maximum temperature over the last 3 hours.

Great, thanks for helping. Appreciated!