The Active Memberships by Date graph begins on the date of your first Membership Transaction. There is no way to adjust this on the report. 

If you'd like to find the date of your first membership transaction, you can follow these steps: 
  1. Go to Analysis > Information Library 
  2. Click Add an ad-hoc query 
  3. Select a Memberships Source View and click OK
  4. In the left column, highlight Membership Transaction and drag Transaction Date (date only) into Results Fields to Display. 
  5. Go to the Set Sort and Group Options Tab and drag Membership Transaction\Transaction Date (date only) into Sort records by and make sure it is sorting Ascending A-Z (oldest to newest). 
  6. Click Preview Results 
  7. You will see the first membership transaction in your database appear first.