allow users to specify explicit tick labels

FixedTicker allows users to specify fixed tick locations, but also need to allow users to specify explicit tick labels

[bryevdv]

Ok I’d like to do something for this for 0.12.6. There’s alot that’s somewhat tangential above, mostly I’d like to return to @pzwang observation that users should not be unduly burdened with considering tickers and formatters unless it’s necessary. So I propose the following:

plot.xaxis.major_label_overrides = { 5: "0-10", 15: "10-20", ... } 

Then internally axis models would pass this on to their tick formatters on init (or when a formatter is replaced). All the built in formatters would refer to this list of overrides (and extension formatters could as well). Users would only need to worry about supplying this mapping on an axis, which is a bit more common/approachable than tickers and formatter.

This would let specific values be overridden, however they occur. If only these ticks are desired, use of FixedTickFormatter would still be needed. However we could probably also make it so that

plot.xaxis.ticker = [5, 15, ... ]

works as a shorthand for setting up a fixed ticker.

Does this sound acceptable to everyone? If so I would plan on working on this after the refactor of categorical values.

[canavandl]

@tommycarstensen , I haven’t tested it, but the below should work:

from bokeh.plotting import show
from bokeh.properties import Dict, Int, String
from bokeh.models import (
    Plot,
    Range1d,
    LinearAxis,
    FixedTicker,
    TickFormatter,
)
from bokeh.util.compiler import CoffeeScript

class FixedTickFormatter(TickFormatter):
    """
    Class used to allow custom axis tick labels on a bokeh chart
    Extends bokeh.model.formatters.TickFormatter
    """

    COFFEESCRIPT =  """
        import {Model} from "model"
        import * as p from "core/properties"
        export class FixedTickFormatter extends Model
          type: 'FixedTickFormatter'
          doFormat: (ticks) ->
            labels = @get("labels")
            return (labels[tick] ? "" for tick in ticks)
          @define {
            labels: [ p.Any ]
          }
    """

    labels = Dict(Int, String, help="""
    A mapping of integer ticks values to their labels.
    """)

    __implementation__ = CoffeeScript(COFFEESCRIPT)

ticker = FixedTicker(ticks=[5,15,25,35,45,55])
formatter = FixedTickFormatter(labels={5: '0-10', 15: '10-20', 25: '20-30', 35: '30-40', 45: '40-50', 55: '50+'})
x_axis = LinearAxis(ticker=ticker, formatter=formatter)

p = Plot(x_range=Range1d(0,60))
p.add_layout(x_axis, 'below')

show(p)