cast getRightValue to number

[kurkle]

1 + '0' evaluates to '10', so stack calculation does not work correctly if values are strings.

This was changed in #4565 where also linear scale was made to cast string to number.
However logarithmic scale uses implementation from Scale where that casting is not done (and can't be since its used for category scale as well)

Wrong size calculation is fixed also per comment by @nagix in original PR.
This calculated size (or center) are currently not used for anything, so that part does not affect any tests.

Fixes: #5862
Replaces: #5892
https://codepen.io/kurkle/pen/KrjXxL

[etimberg]

My only comment is that if we ever document the scale API, we should try and specify that getRightValue returns a number so that we don't need to add in extras to do the conversion

[benmccann]

Would changing scale.getRightValue now be a breaking change? I agree that'd be the nicer way to do it

[simonbrunel]

we should try and specify that getRightValue returns a number

Are we sure it will be always the case? can't we have string values displayed on a category scale? And what about date / time object data for time scale? The return type of getRightValue() depends of the scale type, so this can't be number for all scales. So #5949 doesn't make sense IMO.

Something we should consider in v3 is to move the stacking logic in the scale instead to have each controller to implement a generic method to stack values that works only with numbers.

[simonbrunel]

Thanks @kurkle