The stronger the force of gravity the shorter the time taken for a whole swing (see the time display beneath the graph).
But does the time for the swing halve when the gravity is doubled - or is there some other relationship?

Starting with low gravity (and a low mass - so that increasing gravity doesn' t stretch the pendulum cord much) record the period of the pendulum's swing (ignore any swings during which you've changed the length).
Then increase gravity by 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 gravity and the time taken for its swing?