The given graph is showing cumulative frequency, which is also used to determine percentiles, because this type of frequency shows the percentage representation of the data set with accumulation, that is until completes the whole 100%.
According to the graph, Isa's 9 hours represent 0.6 of cumulative frequency, which means 60%, or percentile number 60.
On the other hand, relative frequencies are useful to show percentiles because a relative frequency indicates the percentage representation of a data set, which is equivalent to percentiles because they are also percentage representation of the data set.
So, basically, you can take relative frequency as the graph od percentiles of a data set.