If you're on an ELITE, MASTER or ENTERPRISE plan (not sure? click here) you may have noticed a little lock icon next to the Stories section under your Analytics tab.
The new Stories analytics tab includes information on how many Stories you've posted within a given time period, the total Reach & Impressions and average Reach & average Reach rate of those Stories. You can adjust the date-picker to get aggregated data on your Stories over a specified period of time. There's also a table that advises you on the best time to post Stories, and data on your average completion rate, and Stories with highest reach, Stories most completed, and Stories with most taps back!
NOTE: As an ELITE, MASTER or ENTERPRISE user, you'll still have the same Stories information as you always had available on the Overview (along with the option to export your Stories data) and on the Stories tab of your 'My Media' feed.