Tap the gear icon in the 'Temp’ / ‘Temp two' card on your data screen (note that you may have renamed these temps).
Then in your temp settings, scroll down a bit and you’ll see the option to enable ‘Annotate temps’ and choose between ‘Circle’ or ‘Triangle’:
When you select 'Annotate today' as an interpretation in the ‘Temp’ / ‘Temp two’ card on your data screen, then the temperature reading for that day will display with a circle or triangle around it on your chart screen.