How many people in the world were living below 15 dollars a day in the last decades? The question I really want to make is if poverty is decreasing of increasing worldwide, but since I guess there is no way to objectively define poverty, that question doesn't fit in here. I was always suspicious that when they release the poverty statistics, they always release the extreme poverty figures and not the poverty figures. For example, recently I found people living under 10 dollars a day either was increasing or it's stagnant (green line, as if they weren't poors). So this leads me to believe poverty is conveniently defined as for showing "poverty is decreasing worldwide". That's why I would like to know if there is a way to figure out how many people leaves under 15 dollars (or any other figure between 10 to 15 dollars, if you like) which according to my country statistics for poverty it would be poverty. By seeing the green and blue lines in this graph the best guess is it's increasing, but I don't have data to sustain that assumption.

So the question is, 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.