Two issues surfaced while verifying #42328's live thumbnail-capture work
against this example chart:
1. Bubble/transformProps.ts set `xAxis.interval` directly from the
xAxisLabelInterval control ('auto'/'0'), but xAxis.interval forces
echarts' IntervalScale into a fixed-tick-spacing mode that expects a
number. This crashed the axis "nice" tick calculation for every
bubble_v2 chart -- not just this example, any bubble chart with the
control at its default. The interval belongs nested under axisLabel,
where it only controls how many labels are skipped, matching how
every other echarts chart in this codebase (e.g. Timeseries) wires it.
There was no existing test coverage for this file; added one.
2. Once the crash was fixed, the chart rendered with zero visible
bubbles: the previous commit's `time_range` (2014, matching the
convention used by sibling examples on this dataset) has zero
non-null SP_DYN_LE00_IN (life expectancy) values in this particular
dataset. The original legacy chart's own choice of 2011 was
deliberate -- that year has real data for all three metrics used.
