New cartesian axis modebarbuttons attribute

[alexcjohnson]

Gives you the ability to disable certain modebar buttons for an axis, without otherwise impacting the interactivity of that axis. This is useful for example if you have one axis with data over a very broad range (like a time series, or in my case, data down a several-thousand-meter drill hole) and the opposite axes have a relatively limited, well-defined range that you don't need to change much.

I made this attribute as a flaglist, mostly for future expansion if needed since there are only two flags right now: auto sets whether the autorange button affects this axis, and inout sets whether the zoom in / zoom out buttons affect this axis. You can also use all (default) and none as shorthands.

[emilykl]

@alexcjohnson It's not entirely clear to me how to think about which buttons are intended to be included/excluded by these options. The most extreme interpretation of, say, inout would seem to be that every other modebar button besides zoomin / zoomout has no effect on that axis -- including e.g. zoom, pan, boxselect. I'm not sure that's a desired behavior (and that's not the behavior of your PR) -- but how can we clarify to the user that buttons like zoom and pan are not affected by this flag? Feel free to let me know if I'm thinking about this the wrong way.

Also a few thoughts on naming —

auto is a bit confusing to me as it makes me think of "automatically choose which modebar buttons affect this axis" rather than "autoscale". How about simply autoscale?

Maybe zoominout rather than inout as well?

[alexcjohnson]

Good point. Maybe this would be clearer as a container with booleans in it, rather than a flaglist? That would also be more extensible, since those booleans could default to true which you can't do with a flaglist. This would work with a flaglist of buttons to disable, but that seems even more confusing.

So perhaps:

modebarbuttons: {
    autoscale: false,
    zoominout: false
}

That's more verbose when you want to disable both, but seems clearer about exactly what it applies to. Thoughts?