Partial bar characters at the start/end of bar plots

[JeroenDelcour]

Currently, bar plots use the full block Unicode character (█) as markers by default. Resolution for the start and end of the bar could be improved by using partial bar characters, e.g. ▀ at the bottom and one of ▁▂▃▄▅▆▇ at the top. Similarly for horizontal bar plots we could use ▐ on the left and one of ▉▊▋▌▍▎▏ on the right.