bokeh_delay_hovertool.py

# Create the blank plot
p = figure(plot_height = 600, plot_width = 600, 
           title = 'Histogram of Arrival Delays',
          x_axis_label = 'Delay (min)]', 
           y_axis_label = 'Number of Flights')

# Add a quad glyph with source this time
p.quad(bottom=0, top='flights', left='left', right='right', source=src,
       fill_color='red', line_color='black', fill_alpha = 0.75,
       hover_fill_alpha = 1.0, hover_fill_color = 'navy')

# Add a hover tool referring to the formatted columns
hover = HoverTool(tooltips = [('Delay', '@f_interval'),
                             ('Num of Flights', '@f_flights')])

# Style the plot
p = style(p)

# Add the hover tool to the graph
p.add_tools(hover)

# Show the plot
show(p)

need to change '@f_flights' to '@flights' in the definition of hover.