Bugfix for fix #1488, #1564

[liuxuan30]

Attempt to fix #1488, #1564
@danielgindi I need you carefully review this and give details about existing implementation about centerAxisLabels feature. My changes may lack of information and cause more issues, though I have tesed it in time line chart, bar chart, combined chart, multiple bar chart, line chart.

1. I removed the dependency of getting initial n value, because first and last will be equal after zooming a lot
2. I change centerAxisLabelsEnabled condition, if entry count > 0, we could turn on centerAxisLabel. I need context why it's > 1.
3. I removed using local variable centeringEnabled, because axis.entries would be changed but centeringEnabled keeps reading old entryCount, which seems a dirty read issue
4. I use offset: Double = interval / 2.0 instead of offset = (axis.entries[1] - axis.entries[0]) / 2.0, because axis.entries[1] - axis.entries[0] always equals to interval, and when there is only one entry, we still need a valid offset instead of 0

[liuxuan30]

@petester42
seeing

*** Building scheme "Realm" in Realm.xcworkspace

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself

But no more logs.. Have you met this?

[pmairoldi]

Ya realm takes too long to build on ci. Usually it's not a problem because the download limit is not reached. Just restart the build.

[danielgindi]

@PhilJay Looks good to me, right?

Well except that var n should be axis.centerAxisLabelsEnabled ? 1 : 0, not 1. Otherwise it misplaces the labels
Also the interval thing should stay as it is - as the increment is not always interval

[danielgindi]

Okay I've done some adjustments and merged