Introduce 'l' as a short form of line

[kojix2]

Currently, there are four sub commands: line, lines, lineplot, and lineplots. In practice, however, lineplot and lineplots are rarely used.

line is relatively long compared to s, the short form of scatter. So I would like to introduce 'l' as a short form of line.

The problem is that there are two types of line: line and lines. We need to consider whether there will be any confusion between the two types of lines.