Use vbar_stack when adding 2 vbars to same plot

[gabalafou]

Fixes #54.

[pavithraes]

Thanks for the PR!

Since we're creating a stacked bar chart, I think it'll be best to use vbar_stack:

from bokeh.plotting import figure, show
from bokeh.models import ColumnDataSource
from bokeh.palettes import HighContrast3

# create ColumnDataSource based on DataFrame from the demo data set
source = ColumnDataSource(monthly_values_df)

# set up the figure
p = figure(
    x_range=source.data["month_name"],  # use the sequence of strings from the "month_name" column as categories
)

# plot values from these columns
columns = ["freight", "passengers", "mail"] 

# use vbar_stack that stacks multiple vbar renderers
p.vbar_stack( 
    columns,
    x="month_name",
    width=0.9,
    source=source,
    color=HighContrast3, # use the HighContrast color palettee
    legend_label=columns,
)

# move legend to left, to avoid overalap with bars
p.legend.location = "top_left"

show(p)

It additionally avoids using bokeh.transform, which is introduced in the following section (after this example).

Alternatively, we can revert this example back to using the line glyph: https://github.com/bokeh/tutorial/blob/01b37f297bd762d28ae0c035dbbe73f8c5b82f93/notebooks/06_data_sources.ipynb

I personally like the stacked bar example -- it presents more useful information, and includes a color palette & legend location change, which were introduced in the previous notebook.

[gabalafou]

That makes sense to me, updated!

[pavithraes]

LGTM, thanks!