Replace DEFAULT_LAST_N in plotting with integer default + add % support

[baggiponte]

DEFAULT_LAST_N as default value is comfy but ugly for the end user: from the docs it's not clear what the default is.

1. Add float support: if 0.1 -> plot 10% of the series
2. Add an heuristic to plot e.g. 10% if it has < 1000 points, 5% if < 10_000 else 1%.

[baggiponte]

Last PR #235 partly addressed this. No weird logics for defaults. Will create a separate issue for % support.