You can use the pencil icon on the top of the ThingSpeak plot to make the timeScale Daily.
Also, you can write the output from the code above to another ThingSpeak channel. We call that a derived channel.
Instead of 'NumPoints',30,
use 'NumDays', 1.
Use thingSpeakWrite() to write the data to you new channel.
And then use a timecontrol to call that code daily. Then you will have a plot of daily average.
Also you could create a matlab visualisation that reads many days of data and then use retime() function to calculate the daily avarage and plot it.
I reccomend using 'outputformat','timetable' when you read the data becasue retime works nicely on timetables.