Hi marvster, not sure if I'm clearly understanding your question, so I'll take a shot at answering and you can let me know if it's what you're after?
I'm assuming you've configured your line chart with your NPS question (numeric value) as the data source and you're then breaking out the results by recorded date (or similar)? Assuming that's correct, then you'll see an option immediately below the "Breakout By" section called "Group Date By". If you select "Day of Week" from this dropdown, then your X axis will show the day of week only (without date).

Hope that helps.
Cheers,
Cameron

Hi Cameron, thanks so much for the response!
I'm using NPS Score under Metrics and Survey Metadata - End Date on the X-Axis, but don't see any Breakout By section like the one shown in your screenshot. Is that supposed to be available in the Simple Chart widget?

Hi marvster, thanks for the clarification. I was incorrectly assuming you were trying to create your line chart via the standard Results/Reports but now realise you're using Simple Chart widgets in a dashboard.
I'm wondering whether you can add Reference Lines to a Simple Chart widget in order to achieve your desired outcome? See this link for more details.
Hope that helps!
Cheers,
Cameron

Thanks, Cameron. I'll take a look!