Splunk has a map command where you can recursively execute SPL for selected field. (https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/Map). I have used makeresults but you can use timechart instead.
<fieldset>
<input type="radio" token="radio" searchWhenChanged="true">
<label>Radio</label>
<choice value="*">All</choice>
<choice value="Dot11Radio0">2.4 GHz</choice>
<choice value="Dot11Radio1">5.0 Ghz</choice>
<default>*</default>
</input>
</fieldset>
<row>
<panel>
<table>
<search>
<query>| makeresults
| eval aggrQuery=case("$radio$"=="Dot11Radio0","avg(low) AS 2.4GHz","$radio$"=="Dot11Radio1","avg(high) AS 5.0Ghz",true(),"avg(high) AS 5.0Ghz avg(low) AS 2.4GHz")
| map search="| makeresults | eval UseMappedresult=\"$aggrQuery$\""</query>
<sampleRatio>1</sampleRatio>
</search>
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