allow users to specify explicit tick labels

[bryevdv]

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

[bryevdv]

One idea would be to add mapping (labels? custom?) to any TickFormatter that maps numeric tick values to specific custom labels for that tick location. Before doing its normal formatting and rendering for a tick value, the TickFormatter would first try to look that value up in the mapping. If it exists, that label is used as-is, instead of the normal formatting.

So use might look something like:

plot.xaxis.formatter.custom[50] = "median"

Random observations:

• this means the customization is attached to a formatter, so if the formatter is changed the customization is lost (potentially confusing in those situations)
• suggests might want ability to add custom fixed ticks to any ticker (or to be able to combine a fixed ticker with a regular ticker)

@bokeh/dev thoughts?

[pzwang]

I can see multiple angles on this. There is an argument to be made that it should be on the Ticker, as well as the Formatter. However, the most important thing IMO is that the split between ticker vs. formatter is an architectural one in our object model, and, under normal circumstances, it's not a distinction that we should burden the user with understanding.

We need to consider the use cases when an end user is most likely to want to specify these things:

1. They have a few, specific locations that they want labelled.
2. They have a custom format that extends an existing built-in formatter
3. They want to replace a built-in ticker+formatter altogether and only show labels at certain locations, regardless of zoom level

What would the ideal spelling look like for each of these? Are there other common use cases I'm missing here?

[Rubyj]

@bryevdv Is there a way to do this yet?

[bryevdv]

Currently you could do this with a custom TickFormatter. Given that there is some path to doing this, even if it is not ideal, adding support into the core library will probably not rise high in the ~750 open issue list. A new interested contributor submitting a PR is probably the quickest route.

[Rubyj]

@bryevdv Thanks. That is currently the way I'm doing it in my code as well.

[canavandl]

Here's an example of doing exactly that (using a FixedTicker and FuncTickFormatter together to set explicit tick labels at specific locations). I think it's the best method for doing this:

from bokeh.plotting import figure, show
from bokeh.models import ColorBar, LinearColorMapper, Plot, Range1d, LinearAxis, FixedTicker, FuncTickFormatter

color_mapper = LinearColorMapper(palette="Spectral6", low=0, high=60)
ticker = FixedTicker(ticks=[5,15,25,35,45,55])
formatter = FuncTickFormatter(code="""
    data = {5: '0-10', 15: '10-20', 25: '20-30', 35: '30-40', 45: '40-50', 55: '50+'}
    return data[tick]
""")

cbar = ColorBar(color_mapper=color_mapper, ticker=ticker, formatter=formatter,
                major_tick_out=0, major_tick_in=0, major_label_text_align='left',
                major_label_text_font_size='10pt', label_standoff=2)

p = Plot(x_range=Range1d(0,1), y_range=Range1d(0,1), width=500, height=500, toolbar_location=None)
p.add_layout(LinearAxis(), 'left')
p.add_layout(LinearAxis(), 'below')

p.add_layout(cbar)

show(p)

[Rubyj]

@canavandl Thank you for the idea. Currently I am using

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

    JS_CODE =  """
        _ = require "underscore"
        Model = require "model"
        p = require "core/properties"
        class FixedTickFormatter extends Model
          type: 'FixedTickFormatter'
          doFormat: (ticks) ->
            labels = @get("labels")
            return (labels[tick] ? "" for tick in ticks)
          @define {
            labels: [ p.Any ]
          }
        module.exports =
          Model: FixedTickFormatter
    """

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

    __implementation__ = JS_CODE

[birdsarah]

@Rubyj I was curious about this, so I put together a complete example.

Please note that you're custom FixedTickFormatter needed to be changed quite a lot for it to work on master (and hence the next release)

from bokeh.plotting import show
from bokeh.properties import Dict, Int, String
from bokeh.models import (
    ColorBar,
    LinearColorMapper,
    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)


color_mapper = LinearColorMapper(palette="Spectral6", low=0, high=60)
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+'})

cbar = ColorBar(color_mapper=color_mapper, ticker=ticker, formatter=formatter,
                major_tick_out=0, major_tick_in=0, major_label_text_align='left',
                major_label_text_font_size='10pt', label_standoff=2)

p = Plot(x_range=Range1d(0,1), y_range=Range1d(0,1), width=500, height=500, toolbar_location=None)
p.add_layout(LinearAxis(), 'left')
p.add_layout(LinearAxis(), 'below')
p.add_layout(cbar)
show(p)

@bryevdv - we're going to need some major migration notes about the new export stuff, then new import stuff, and the compilation stuff for folks