Pin matplotlib version in requirements.txt or drop usage of hold to avoid AttributeError

[jhadjar]

Summary:

---

Running the notebook ipynb/Economics.ipynb raises an AttributeError exception with matplotlib-3.1.3 installed.

Reproduce:

---

• Run the notebook with a matplotlib version that deprecated hold

Actual outcome:

---

An AttributeError exception is raised.

Expected outcome:

---

A sweet notebook running.

Logs &/or traceback:

---

Traceback

AttributeError Traceback (most recent call last)
 in 
----> 1 show(population)
 in show(population, k, percentiles, **kwargs)
19     plt.title('/'.join(map(str, percentiles)) + ' Percentile Plots')
20     times = [t for (t, pop) in results]
-->21     plt.hold(True); plt.xlabel('wealth'); plt.ylabel('time'); plt.grid(True)
22     for pct in percentiles:
23         line = [percent(pct, pop) for (t, pop) in results]
AttributeError: module 'matplotlib.pyplot' has no attribute 'hold'

Likely parts and draft fix:

---

Change requirements.txt to pin the matplotlib version to something like 2.1.0:

numpy
matplotlib==2.1.0

Another option is to update the code in ipynb/Economics.ipynb to use the updated API.