The longer the length of the pendulum the longer the time taken for a whole swing (see the time display beneath the graph).
But does the time for the swing double when the length is doubled - or is there some other relationship?

Starting with a short length, record the period of the pendulum's swing (ignore any swings during which you've changed the length).
Then increase the length by a factors of 2, 3, 4, 8 & 9 and record by what factor each changes the time of the swing.

So what exactly is the relationship between the length of the pendulum and the time taken for its swing?