Bokeh slider threshold

from itertools import product

import pandas as pd
from bokeh.io import show, output_file
from bokeh.layouts import column
from bokeh.models import CustomJS
from bokeh.models.sources import ColumnDataSource
from bokeh.models.widgets import Slider
from bokeh.plotting import figure

N = 30
INITIAL_STEP = 0

data = []
for r, c in product(range(N), range(N)):
    step = max(r, c)
    data.append(dict(x=c, y=r,
                     infected_by_x=(c - 1 if c > 0 else c),
                     infected_by_y=(r - 1 if r > 0 else r),
                     infected_step=step,
                     color='blue' if step > INITIAL_STEP else 'red'))

data = pd.DataFrame(data)
ds = ColumnDataSource(data)

line_data = []
for row in data.itertuples():
    step = max(row.x, row.y)
    line_data.append(dict(xs=[row.infected_by_x, row.x],
                          ys=[row.infected_by_y, row.y],
                          infected_step=row.infected_step,
                          alpha=0 if step > INITIAL_STEP else 1))
line_data = pd.DataFrame(line_data)
line_ds = ColumnDataSource(line_data)

slider = Slider(start=0, end=N - 1, value=INITIAL_STEP)
slider.js_on_change('value', CustomJS(args=dict(dots_ds=ds, lines_ds=line_ds),
                                      code="""
    var update_status = function(ds, field, healthy, infected) {
        for (var di = 0; di < ds.data[field].length; ++di) {
            ds.data[field][di] = (ds.data['infected_step'][di] > cb_obj.value) ? healthy : infected;
        }
        ds.change.emit();
    };
    update_status(dots_ds, 'color', 'blue', 'red');
    update_status(lines_ds, 'alpha', 0, 1);
"""))

f = figure()
f.circle(x='x', y='y', color='color', source=ds)
f.multi_line(xs='xs', ys='ys', color='red', alpha='alpha', source=line_ds)

output_file('test.html')
show(column(f, slider))