split series Y-Axis custom labeling doesn't work well in the tooltip

[ppisljar]

Kibana version:
7.0

Describe the bug:

Horizontal Bar Y-Axis custom labeling doesn't work well in the tooltip when we use "Split Series" int the Buckets as sub aggs.

Y-Axis cutom label is overwritten by the sub aggs key value. so the customer may not be able to see the Y-Axis value if sub aggs key value has very long value like system.process.cmdline.

this behavior doesn't happen in 6.5.4. so I think this issue may be bug..

Expected behavior:
in 6.5 we can set custom label correctly:

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-app