Indicate current zoom direction, restrict options

[sboeser]

It seems there is a default zoom direction, that can be changed by pressing the Ctrl-key once. However, there is no indication as to what the current zoom direction is.

• add a visual indicator that shows which axis will be zoomed in current status
• allow to restrict the possible zoom directions when generating the plot

Btw: after playing around with this for a while, I am still in favour of the "ROOT-style" zooming, where you drag over a range on the x- or y-axis to zoom in. This allows more precise control over the region that one zooms into, in particular if you get an indicator (e.g vertical/horizontal lines) that shows the range while you zoom. Unzooming is not obvious in this scheme, though...

[sboeser]

https://bugs.icecube.wisc.edu/view.php?id=6812