The data obtained in experiments should be recorded in tables and turned into graphs to be able to better study their relations. 

Galileo probably collected his data like you are going to do in the following experiment:

Start the visual (click on Pendulum)  and note down the data obtained in the table on the left. 
The independent variable is usually placed in the first column. Remember that the period is the time taken to make a complete oscillation (for example the time it takes to get back to the initial position). When you write the values in the table, the pendulum disappears, click on  Pendulum again to recover it. 
Which are the dependent and independent variables here?