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I am using Bokeh 1.0.1. I am unable to update the data source in the Update method i.e src.data.update(new_src.data) doesn't seem to work. Below is the full code.

def modify_doc(doc):

 def create_dataset(df, resample='D'):
 # Resample the data
 src = df.resample(resample).mean()

 # Reset index for hovering
 src.reset_index(inplace=True)

 return ColumnDataSource(src) 

 def create_plot(src):
 # Blank plot with correct labels
 p = figure(plot_width=700, plot_height=300, x_axis_type="datetime",
 title = 'Variation of Pollution',
 x_axis_label = 'Time', y_axis_label = 'Pollution (µg/m³)')

 p.line(source=src, x='Date & Time', y='pm2.5', line_width=2,
 color='firebrick', line_alpha=0.5, legend='Actual')

 hover = HoverTool(tooltips=[('Pollution', '@pm2.5 µg/m³'),
 ('Air Temp', '@Air Temp. °C'),
 ('Temp', '(@Min. Temp.0.2f, @Max. Temp.0.2f) °C'),
 ('Dew Pt.', '@Dew Pt. °C'),
 ('Rainfall', '@Rainfall mm'),
 ('Wind Dir.', '@Wind Dir. °'),
 ('Wind Speed', '@Wind Speed km/hr'),
 ('Relative humidity', '(@Min. RH0.2f, @Max. RH0.2f) %')],
 mode='vline')

 p.add_tools(hover)
 p.legend.click_policy = 'hide'

 return p

 # Update function takes three default parameters
 def update(attr, old, new):
 # Resampling list
 re_list = ['D', 'W', 'M', 'A']

 # Make a new dataset based on the selected carriers and the 
 # make_dataset function defined earlier
 new_src = create_dataset(df,
 resample = re_list[resample_button_group.active]) 


 # Update the source used the quad glpyhs
 src.data.update(new_src.data)



 resample_button_group = RadioButtonGroup(labels=["Day", "Week", "Month", "Year"], active=1)
 resample_button_group.on_change('active', update)

 controls = WidgetBox(resample_button_group)

 # Initial Plot
 src = create_dataset(df)
 p = create_plot(src.data)

 layout = row(controls, p)
 doc.add_root(layout)

# Set up an application
handler = FunctionHandler(modify_doc)
app = Application(handler)

You should be able to update the line glyph directly.

First, modify your plotting code to assign a name to the line glyph:

pm_line = p.line(
 source=src,
 x='Date & Time', 
 y='pm2.5', 
 line_width=2,
 color='firebrick', 
 line_alpha=0.5, 
 legend='Actual',
 name='pm_line' # Add this!
)

Then in your update function, replace your existing update line with the following:

pm_line = p.select_one('name':'pm_line')
pm_line.data_source.data = new_src.data