setup.py: add matplotlib to 'plot' extra, used by varLib.plot

README.rst

@@ -245,6 +245,14 @@ are required to unlock the extra features named "ufo", etc.
 
   *Extra:* ``interpolatable``
 
+- ``Lib/fontTools/varLib/plot.py``
+
+  Module for visualizing DesignSpaceDocument and resulting VariationModel.
+
+  * `matplotlib <https://pypi.org/pypi/matplotlib`__: 2D plotting library.
+
+  *Extra:* ``plot``
+
 - ``Lib/fontTools/misc/symfont.py``
 
   Advanced module for symbolic font statistics analysis; it requires:

setup.py

@@ -65,6 +65,12 @@ extras_require = {
 		"scipy; platform_python_implementation != 'PyPy'",
 		"munkres; platform_python_implementation == 'PyPy'",
 	],
+	# for fontTools.varLib.plot, to visualize DesignSpaceDocument and resulting
+	# VariationModel
+	"plot": [
+		# TODO: figure out the minimum version of matplotlib that we need
+		"matplotlib",
+	],
 	# for fontTools.misc.symfont, module for symbolic font statistics analysis
 	"symfont": [
 		"sympy",