Respone Correlation Viewer: Fuzzy click selection of observations

[valentin-krasontovitsch]

Is your feature request related to a problem? Please describe.
Sometimes there are many datapoints in the response overview plot, and looking at a response with observations, it might be difficult to click an observation to choose it without zooming in.

Consider the following snapshot of a response overview plot:

raised by @oysteoh in a comment on a PR

Describe the solution you'd like
One might make the click selection fuzzy in the sense that if a user clicks not exactly on, but close to an observation point, the observation is selected as active x index.

for thoughts on implementation, cf. another comment on the same PR

Describe alternatives you've considered
An alternative that makes this feature a bit superfluous / a bit more of just a "nice" thing to have is the implementation of an observation selector, which implements its own selector element for observations - depending on the final form of that selector, a better user experience with the graphic selection method might still be desirable.

Additional context
One should consider that users should still have the possibility to choose a point close to an observation.