Improvements to binary state viewer

Suggested improvements to https://github.com/Nikea/xray-vision/blob/master/examples/plotting/binary_lines.py:
- add a key with which color corresponds to 0/1 or low/high or on/off. I have no preference in terms of labeling
- have the 'y' value in the lower left corner of the window show the corresponding value. It is currently blank `(x=15.8949 y=)`
- add a time axis at the bottom
