Task "realWork" history

Hello,

for some reports and help on decision I would like to create a report that is giving all the changes on the "realWork" information of some activities (generally some summary activities).
The scope is to have a graph like the as attached 



I though to use the API to retreive this information (as I didn't find a direct report for it)

I can find the current value of the realWork on the "Activity" element, but I do not know how to retreive all the changes on the History elements, even if I can see them on the History tab of the tool.

Is anybody has some hints to retrieve this information?

Thank you,