* Feature: Flexible Y-Min and Y-Max settings.

Y-Min and Y-Max is now string.
New usage for both Y-Min and Y-Max (Where X is a real number):
  >X Y-Max/Y-Max is auto if data is above X else X
  <X Y-Max/Y-Min is auto if data is below X else X
  =X Y-Max/Y-Min is scaled to current value +/- X
  ~X Y-Max/Y-Min is caled to average value +/- X

Example: Y-Min: <100 Y-Max: >200
If all points are within 100 and 200: Y-Min is 100 and Y-Max is 200
If some points are above 200: Y-Min is 100 and Y-Max is auto
If some points are below 100: Y-Min is auto and Y-Max is 200
if some points are below 100 and above 200: Y-Min and Y-Max is auto

Tests for new settings added

* Use parseFloat instead of implicit conversion

* feat(flexible_y-min/max): refactor.

* feat(flexible_y-min/max): added more tests.