You could do a series of successive filters using the "Created At" field, specifying timeframes.
1. Created At - On or Before 12/31/2012
2. Created At - On or Before 1/31/2013
3. Created at - On or Before 2/28/2013
etc.
This will provide only known leads.
You could do the same thing for Anonymous leads, but you need to drag the "Is Aonymous filter" and select true, and use all filters. Keep in mind that anonymous data is purged after three months if the lead has not maintained activity with your webisite.
If you want the combined total, you need to drag the "Is Anonymous" filter twice, and set it once to true and once to false (and use 3 filters, selecting Combined filters--[ (1. Created on or before date) and (2. anon=true or 3. anon=false) ]