Improve null and undefined checking to allow valid values like 0
Allow constructor to take in options and merge with defaults
Improve documentation
Sets derived properties of axes from defaults
Use defaults/options and derived properties to draw graph
Refactor axes drawing to use options/defaults, derived properties, and a separate method
Add and use default stroke color for axes
Refactor graph drawing method to use defaults/options
Reformat method for clarity and style
Simplify graph drawing function by setting values in constructor