The complete API documentation is available either online, or as part of the full package in the download section. The package contains the documentation in the form of an HTML hierarchy (the same you access online) and as a qch help file for QtCreator/Assistant integration. If you use QtCreator or Assistant, you should definitely consider using the qch file, it will greatly improve your workflow!
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.
That's it! Now, when you place the cursor on any QCustomPlot related class or function, press F1 and the corresponding documentation entry pops up. Just like you know it from Qt components.