Changing Date Histogram interval from Hourly to Auto introduced false data points for stacked graphs.

[mykael22000]

Kibana version: 6.4.3

Elasticsearch version: 6.4.3

Server OS version: Window 10

Browser version: Firefox 64.0.2

Browser OS version: Windows 10

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

Describe the bug: The combination of Date Histogram interval: Auto and graph style: stacked introduces false data points into my graph. I have hourly data. Line graphs look fine with interval AUTO, but if I make it a stacked graph, I get a whole bunch of false data points introduced on every half hour.

Steps to reproduce:

1. Get some hourly data (only datapoints on each hour)
2. Visualizes them as a line graph with x-axis Date Histogram Interval: Auto (all looks ok).
3. Change the graph type to stacked (on the metrics and axies page) The graph now has a bunch of 0 data points on the half hour added, giving an exaggerated sawtooth effect.
4. Change the interval to hourly to fix it.

Expected behavior: No false data points are introduced

Screenshots (if relevant): The line graph:

The incorrect graph after setting the interval back to auto:

Errors in browser console (if relevant): None.

Provide logs and/or server output (if relevant): None.

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-app