Proposed API for twin-axis support

Twin axes is an often requested feature (e.g. see #396, #1594, #3011) and HoloViews should support this type of plot without having to resort to ugly custom hooks.

In this issue I would like to propose an API for discussion. In particular, I think the most intuitive approach would be to support twin-axes by creating a simple overlay, where one of the components has yaxis='twin' specified as a plot option. Right now, the possible values for yaxis (for bokeh) are ['left', 'right', None, True, False] which means 'twin' would be added to this list. For matplotlib, the options are ['left', 'right', None] (I don’t see why true and False shouldn’t also be supported here though!)

As 'left' and 'right' are already supported, using 'twin' would add axes on the opposite side for the component it is attached to. For instance:

1. curve1 * curve2.opts(yaxis='twin'): curve1 has its yaxis on the left (default) and curve2 has its yaxis on the right
2. curve1.opts(yaxis='right') * curve2.opts(yaxis='twin'): curve1 has its yaxis on the right and curve2 has its yaxis on the left

I think this would be intuitive enough given how everything currently works. In principle, this should support overlaying two NdOverlays so that you have two legends, one for each side.

Suggested rules:

1. The x-axis dimension must match otherwise trying to use twin-axes should be an error.
2. Twin axes make sense for a number of chart elements (e.g. curves, scatter) but I don’t think they really make sense for Points and Paths which exist in a homogenous 2D space (where you can sensibly imagine rotations). I don’t think this option will make sense for all element types (e.g. can this really be sensible for bar charts?)

[douglas-raillard-arm]

It’s maybe a terrible idea but it just struck me that we actually can have an unlimited supply of infix operators in Python if we are ready to make pylint scream about the missing whitespaces:

class Meta(type):
    def __rlshift__(self, left):
        return self(left)

class Op(metaclass=Meta):
    def __init__(self, left):
        self.left = left

    def __rshift__(self, right):
        return self.op(self.left, right)


class Custom(Op):
    @staticmethod
    def op(left, right):
        return left * right

# We just created a new operator "<<Custom>>"
print(3 <<Custom>> 2)

[maximlt]

Allowing twin axes makes sense too for two elements that are not of the same type. E.g. in this example taken from Microsoft’s site: https://support.microsoft.com/en-us/office/add-or-remove-a-secondary-axis-in-a-chart-in-excel-91da1e2f-5db1-41e9-8908-e1a2e14dd5a9#OfficeVersion=macOS