How to control the number of stacked bars through single select widget in python bokeh - python-3.x

Question

I have created a vertical stacked bar chart using python bokeh on an input dataset df using the following code -
print(df.head())
YearMonth A B C D E
0 Jan'18 1587.816 1586.544 856.000 1136.464 1615.360
1 Feb'18 2083.024 1847.808 1036.000 1284.016 2037.872
2 Mar'18 2193.420 1850.524 1180.000 1376.028 2076.464
3 Apr'18 2083.812 1811.636 1192.028 1412.028 2104.588
4 May'18 2379.976 2091.536 1452.000 1464.432 2400.876
Stacked Bar Chart Code -
products = ['python', 'pypy', 'jython']
customers = ['Cust 1', 'Cust 2']
colours = ['red', 'blue']
data = {
'products': products,
'Cust 1': [200, 850, 400],
'Cust 2': [600, 620, 550],
'Retail 1' : [100, 200, 300],
'Retail 2' : [400,500,600]
}
source = ColumnDataSource(data)
# Set up widgets
select=Select(options=['customers','retailers'],value='customers')
def make_plot() :
p=figure()
#p.title.text=select.value
if select.value=='customers' :
customers=['cust 1','cust 2']
else :
customers=['Retail 1','Retail 2']
p.hbar_stack(customers, y='products', height=0.5, source=source, color=colours)
return p
layout = column(select, make_plot())
# Set up callbacks
def update_data(attrname, old, new):
p = make_plot() # make a new plot
layout.children[1] = p
select.on_change('value', update_data)
# # Set up layouts and add to document
curdoc().add_root(layout)
Now I want to limit the number of segments(ie.stacked bars) by using a widget (preferrably by a single select widget). Can anyone please guide me how can i achieve using bokeh serve functionality. I don't want to use Javascript call back function.

Answer 1

This would take some non-trivial work to make happen. The vbar_stack method is a convenience function that actually creates multiple glyph renderers, one for each "row" in the initial stacking. What's more the renderers are all inter-related to one another, via the Stack transform that stacks all the previous renderers at each step. So there is not really any simple way to change the number of rows that are stacked after the fact. So much so that I would suggest simply deleting and re-creating the entire plot in each callback. (I would not normally recommend this approach, but this situation is one of the few exceptions.)
Since you have not given complete code or even mentioned what widget you want to use, all I can provide is a high level sketch of the code. Here is a complete example that updates a plot based on select widget:
from bokeh.layouts import column
from bokeh.models import Select
from bokeh.plotting import curdoc, figure
select = Select(options=["1", "2", "3", "4"], value="1")
def make_plot():
p = figure()
p.circle(x=[0,2], y=[0, 5], size=15)
p.circle(x=1, y=float(select.value), color="red", size=15)
return p
layout = column(select, make_plot())
def update(attr, old, new):
p = make_plot() # make a new plot
layout.children[1] = p # replace the old plot
select.on_change('value', update)
curdoc().add_root(layout)
Note I have changed your show call to curdoc().add_root since it is never useful to call show in a Bokeh server application. You might want to refer to and study the User Guide chapter Running a Bokeh Server for background information, if necessary.