When adding a new chart, you'll first be asked to configure the source settings. The source is simply referring to the location of the data you would like to analyse.
For Frequency & Ratio Charts:
Select the Template
This will be the template you would like to draw the data from.
For Quantity Charts:
Select the Template
This will be the template you would like to draw the data from.
Note: For Quantity Charts, only templates with pre-filled/default tables including number/formula fields, will be visible on this dropdown.
Select the Field
This selects the specific table, from within the chosen template, that you would like to analyse.
Select the Column/Cell
This will display data from that chosen Column/Cell from within the table.
Note: You can change the selected source/template later on, if you need. This will keep any other settings/filters you have already configured (for Quantity Charts, you would just need to reconfigure the Field/Column/Cell)