Make installation optional

Author: ColCarroll

README.rst

@@ -78,6 +78,13 @@ Or via conda-forge:
 
     conda install -c conda-forge pymc3
 
+Plotting is done using `ArviZ <https://arviz-devs.github.io/arviz/>`__
+which may be installed separately, or along with PyMC3:
+
+::
+
+    pip install pymc3[plots]
+
 The current development branch of PyMC3 can be installed from GitHub, also using ``pip``:
 
 ::

pymc3/plots/__init__.py

@@ -1,3 +1,9 @@
+"""PyMC3 Plotting.
+
+Plots are delegated to the ArviZ library, a general purpose library for
+"exploratory analysis of Bayesian models." See https://arviz-devs.github.io/arviz/
+for details on plots.
+"""
 try:
     import arviz as az
 except ImportError:  # arviz is optional, throw exception when used
@@ -8,7 +14,7 @@ def __init__(self, attr):
 
         def __call__(self, *args, **kwargs):
             raise ImportError(
-                f"ArviZ is not installed. In order to use `{self.attr}`:\npip install arviz"
+                "ArviZ is not installed. In order to use `{0.attr}`:\npip install arviz".format(self)
             )
 
     class _ArviZ:

requirements-dev.txt

@@ -1,10 +1,10 @@
+arviz
 bokeh>=0.12.13
 CommonMark==0.5.4
 graphviz>=0.8.3
 h5py>=2.7.0
 ipython
 Keras>=2.0.8
-matplotlib>=1.5.0
 nbsphinx>=0.2.13
 nose>=1.3.7
 nose-parameterized==0.6.0

setup.py

@@ -60,5 +60,8 @@ def get_version():
           include_package_data=True,
           classifiers=classifiers,
           install_requires=install_reqs,
+          extras_require={
+              "plots": ["arviz"],
+          }
           tests_require=test_reqs,
           test_suite='nose.collector')