The integration of the qch file is pretty straight forward: Copy the qcustomplot.qch file to a place where it should be stored (e.g. the local QtCreator config directory). In QtCreator, go to the program settings and find the Help section. In the tab Documentation, you see a list of loaded documentation modules and some buttons to add/remove modules. Click the Add... button and select the qcustomplot.qch file in the appearing dialog.
with the qcustomplot.h header provided by the qcustomplot-qt5 AUR package. This gives the compilation error (in Geany, I know the first line is Geany's attempt at compiling example.cpp and producing the example binary):
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