Revise Calculator decile_graph method to use new distribution table

[martinholmer]

This pull request revises the Calculator.decile_graph method to allow graphing of the bottom decile to include or not those filing units with zero expanded_income and to include or not those filing units with negative expanded_income. This approach leaves it up to the Tax-Calculator user how to display the percentage change in after-tax expanded income for the bottom decile in the graph.

Given the new method, the following script (which uses the same reform as in #1917) produces the following four graphs:

from __future__ import print_function  # necessary only if using Python 2.7
from taxcalc import *

# select either CPS data or PUF data
recs = Records.cps_constructor()
# recs = Records()

# specify Calculator object for static analysis of current-law policy
pol = Policy()
calc1 = Calculator(policy=pol, records=recs)

cyr = 2020

# specify Calculator object for static analysis of adding a tax-exempt UBI
reform = {cyr: {'_UBI_u18': [1e4], '_UBI_1820': [1e4], '_UBI_21': [1e4],
                '_UBI_ecrt': [1.0]}}
pol.implement_reform(reform)
if pol.reform_errors:  # check for reform error messages
    print(pol.reform_errors)
    exit(1)
calc2 = Calculator(policy=pol, records=recs)

# calculate baseline and reform Calculator objects
calc1.advance_to_year(cyr)
calc1.calc_all()
calc2.advance_to_year(cyr)
calc2.calc_all()

# generate decile graph for cyr
fig = calc1.decile_graph(calc2,
                         include_zero_incomes=True,
                         include_negative_incomes=True)
write_graph_file(fig, 'graph.html', 'DECILE GRAPH')

Here is the graph when using CPS input data and including all the zeros and negatives in the bottom decile:

Here is the graph when using CPS input data and excluding all the zeros and negatives in the bottom decile:

Here is the graph when using PUF input data and including all the zeros and negatives in the bottom decile:

And here is the graph when using PUF input data and excluding all the zeros and negatives in the bottom decile:

[codecov-io]

Codecov Report

Merging #1918 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1918   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3605    3625   +20     
======================================
+ Hits         3605    3625   +20

Impacted Files	Coverage Δ
taxcalc/utils.py	100% <100%> (ø)	⬆️
taxcalc/calculate.py	100% <100%> (ø)	⬆️

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[MattHJensen]

LGTM. Thanks very much @martinholmer and everyone who participated in the discussion that led here!