With a good set of results the analysis is straightforward. The first graph of time against length shows a nice smooth curve. The shape of the curve indicates a relationship such as square root, or cube root. In this case a square root relationship works very well. Note that it is not always possible to find a simple relationship like this.
It is not difficult to look up the formula relating time and length;
T = 2π√(L/g)
Putting in the numbers for π (3.14) and g (10N/kg) gives;
T = 2.01√L
(where L is in metres)
So we can see that our results are in agreement. In the analysis the results have been worked through in centimetres. Converting to the SI unit of metres makes no difference to the conclusions although some people prefer to work with "standard" units.
When carried out this way the simple pendulum is so straightforward that it is not really much of an investigation. All we are doing is testing out a textbook formula.