Grid for CategoricalAxis group and subgroup levels

Hello,

I cann’t find any mechanism to add grid for CategoryAxis group and subgroup levels.

Is there an idiomatic (“Bokehic”) way to solve the problem? Or is this a reason to open a new feature request?

Empirically, I have chosen a formula for calculating (emulating) grid for group-level like

x=[(lhue + 1.4)*i - 0.7 for i in range(lgroup+1)]

But it is not “infinite” (it is defined and does not redraw when the scale changes), unlike “regular” grids.

See example below

from bokeh.models import ColumnDataSource, FactorRange
from bokeh.plotting import figure, show

fruits = ['Apples', 'Pears', 'Nectarines', 'Plums', 'Grapes', 'Strawberries']
years = ['2015', '2016', '2017']

data = {'fruits' : fruits,
        '2015'   : [2, 1, 4, 3, 2, 4],
        '2016'   : [5, 3, 3, 2, 4, 6],
        '2017'   : [3, 2, 4, 4, 5, 3]}

# this creates [ ("Apples", "2015"), ("Apples", "2016"), ("Apples", "2017"), ("Pears", "2015), ... ]
x = [ (fruit, year) for fruit in fruits for year in years ]
counts = sum(zip(data['2015'], data['2016'], data['2017']), ()) # like an hstack

source = ColumnDataSource(data=dict(x=x, counts=counts))

p = figure(x_range=FactorRange(*x), height=350, title="Fruit Counts by Year",
           toolbar_location=None, tools="")

p.vbar(x='x', top='counts', width=0.9, source=source)

p.y_range.start = 0
p.x_range.range_padding = 0.1
p.xaxis.major_label_orientation = 1
p.xgrid.grid_line_color = None

# create grid for group-level
lhue = len(years)
lgroup = len(fruits)

p.vspan(
    x=[(lhue + 1.4)*i - 0.7 for i in range(lgroup+1)],
    line_width=1, line_color="lightgray",
)

show(p)

bokeh_plot(4)837×488 16.7 KB

(based on Bar charts — Bokeh 3.3.2 Documentation example)

Bokeh does not currently display any “grid” along categorical axes, only group level separators when there are multi-level categories in use (as above). You can definitely submit a GitHub Issue to request this feature for future development, though I can’t speculate when anyone might look at it.

@Bryan Could you please provide a formula/algorithm or a place in the code that will make it clear how the level separators are formed in the X-axis coordinates.

Group separators are drawn here:

using tick coordinates computed here:

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