I've started to write an application in c++ that uses libplotter. I like
very much capabilities of this library and the fact that I don't need to
know anything about X window to get a window with a hi-quality plot.
However, I've realized that it'd be cool being able to, say, change
scale of a plot after clicking on it, or react on keyboard events to
change parameters of a plot, etc.

In what direction should I go?

As far as I've googled, I need some widgets. But I know nothing about
widgets, and hence the question: which widgets
for c++ are simple to learn and capable of working with plotutils?

There is an example in plotutils documentation of using Xt, but I failed
to google out anything desicive about using Xt with C++ and I definitely
want to avoid using C.

Best regards,
Lukasz Grabowski