fix: getEntityState() for "last" & "bar"

[ildar170975]

Currently using "last" & "bar" causes a card to be not rendered due to an error.

type: custom:mini-graph-card
entities:
  - entity: sensor.system_monitor_processor_use
show:
  state: last
  graph: bar

It happens because "this.points" is not defined if graph type = "bar".

After the fix:

[ildar170975]

So we have 3 PRs to fix the same problem:

1. fix: always get points so state last works with other graphs #1075 - "always define points".
2. fix: state: last to use value from last Graph coordinate #1308 - "use coords"
3. this one - "ether use points or bars".

Currently "points" & "bars" already have properly (I hope so) defined values.
So why not using these data instead?

Imho:

1. Calculating "points" in case of bars is too "artificial".
2. Using "coords" in "main()" is too "low level" because "coords" are inside a Graph object, I do not think we are supposed to use them directly in "main()".

[akloeckner]

Imho:

On the conceptual level, I'd agree. I'm wondering: will this fix also work, if show.points is false? (I think, that is a valid option, right?)

[ildar170975]

I'm wondering: will this fix also work, if show.points is false? (I think, that is a valid option, right?)

@akloeckner
Again, sorry for a delay.

1. A "point" term is only applied to a "line" graph, not to a "bar".
2. IMHO using together graph: line, show_points: false & state: last is wrong ))).
   See, the state: last means "show me a value of the last point", and by "points" we mean dots displayed on a curve.
   Since a curve is build using some algorithm, the "last point" in general could be not a "current state". In fact, any "point" may not contain a REAL value, only some interpolated one.
   (proposed some clarifications for Docs - docs: Add a remark regarding points #1346)
   So, if a user defined together graph: line, show_points: false & state: last - I would fallback to state: true, i.e. showing a current state.
   Added a commit for this fallback.

[ildar170975]

Additional changes:

1. Added descriptions for getObjectAttr() & getEntityState() methods.
2. In getEntityState(), an input argument id is renamed to index to be more consistent with other places.

[ildar170975]

Related - #1083