How many people in the world were living below 15 dollars a day in the last decades?
Most of the "official" statistics compiled in international organisations use some thresholds like the one shown in the graph. Common for instance are the one by the World Bank (data here and report here). 

Therefore, unless you find such data already computed, it is not possible to tell. 

There is one imprecise alternative though. You can use this online tool to compute global poverty based on any threshold you like (using annual income in £). For example, 15 US dollars correspond to roughly to £11.64 (as of 31/08/17, 13:25 BST), which amounts to a yearly (365 days) income of £4,250, which yields:



As the FAQ explains, this analysis is based on data from the World Bank, is valid for 2008, and assumes a population of 6.69 billion. Therefore, based on the above result, 5.34 billion people lived below $15 a day in 2008. This is "sort of" consistent with the data in the graph you posted, in my opinion.

Unfortunately, such analysis is only valid for 2008.