When on the Dashboard tab of the AS UI, you will see several reports with data plotted in a line graph. If you click the gear icon in the lower-right corner of any report, you will have the ability to configure what each report displays. Choose the Attribute, Audience, Action, Omnichannel, etc to be reported, the time period over which the data should be reported, then click Finish to see the results. Be sure to Save/Publish the changes so that they are available to other users.
If you are looking to trend just Visitors, then under "Daily Visitors" select "All Visitors", which should be the first option to select.