[FR] Add a "stepline" transition

[ildar170975]

Currently 2 points are connected by a line (which can also be smoothed).
In some cases it gives a wrong impression about a state between these points:

Here red points are generated by mini-graph-card based on actual readings from DB.
A blue point is located between two red points.

Proposal: add a "stepline" transition (like in a standard history-graph):

This option should not be used together with smoothing: true.
Also, this option should be per-entity (and also can be defined on a card's level globally for all entities; per-entity definition has a priority).