Hej AIMPED
thank you for your suggestion!,
In fact my current workaround is manually switching the 'dragmode': 'drawopenpath' to 'dragmode': 'zoom' upon clicking an extra-button. Following Clicking on the Graph, all the required settings are made, and the dragmode is switched back to 'dragmode': 'drawopenpath'.
Another option would have been to use HoverData (which is working regardless of the dragmode-setting) and to display the required information to the user, such that the user is able to make the desired settings him/her-self. However, I feel this is less convenient and requires more space for displaying the information.
Note: I am still interested why 'dragmode': 'drawopenpath' (and other ‘draw’.modes) is incompatible with ClickData, since no relayout event is fired upon clicking the Graph, either.