Add plotting support for unbounded intervals

[omus]

The PR which added support for unbounded intervals did not add support for plotting them.

It was briefly looked into and the main issue is that an unbounded interval should print for +/- infinity or typemax / typemin but from some initial experimentation it appears that lines created this way are too large and are just not plotted at all.

The other less pressing issue is that we need a different marker to display an unbounded endpoint. My initial thoughts were to use no marker for unbounded and use custom markers to create shapes that look like square bracket and parenthesis to represent closed/open intervals.