Show X-axis lines under grid doesn't show any if x-axis is not drawn on date field

[bhavyarm]

Kibana version: 7.7.0 BC6

Elasticsearch version: 7.7.0 BC6

Server OS version: darwin_x86_64

Browser version: chrome latest

Browser OS version: OS X

Original install method (e.g. download page, yum, from source, etc.): from staging

Describe the bug: If user creates a xy chart and uses a non date field on x-axis to draw the chart - then checking the option "Show X-axis lines" under grid in panel settings doesn't show the x-axis lines

Steps to reproduce:

1. Click on create visualization and pick a bar chart and choose any sample data index pattern
2. Add a term aggregation for x-axis
3. Go to panel settings and enable showing grid lines over x-axis
4. It doesn't show

[elasticmachine]

Pinging @elastic/datavis (Team:DataVis)

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[darshakkakkad]

Hi, do you have any update on this? I see the same issue(Kibana 7.7.1) where checking/unchecking the "X-Axis Lines" has no effect. The grid lines never show up for X-Axis.