The Sign-in Report is a little bit different. You can access it by going to one of the Learners reports and switching using the top dropdown, or you can click the pictured Deep Dive button on the Insights page:
Let's take a look at this report:
As you can see, we're looking at logins from the Last Month. You can change this timeframe in the upper left.
The gray bars show you how many users logged in on each given week of the report. The purple line breaks that down to show how each day of that week compared to other days. You can turn this off by clicking the "yes" next to Show Daily Values.
Finally, you can export a CSV of this report, which will show you how many people logged in each day for the time represented on the report.