Stacked chart doesn't load (displays N/A)

[Tastyep]

Hello,

After upgrading my version of graylog to 3.0, I noticed that stacked charts failed to load properly.

Current Behavior

Loading the stacked chart from a content pack fails with the following error:
There was an error fetching a resource: cannot GET http://172.19.0.5:9000/api/dashboards/5c7d40573ecd49000dbc23a6/widgets/8119956c-682d-4f7b-913f-484a1560b3c4/value (500). Additional information: Null from
The widget loads, but its content is just a text equal to N/A.

Steps to Reproduce (for bugs)

I don't have a live example, but the problem is fairly easy to reproduce.

1. Go on a stream and generate two charts
2. Merge the two charts
3. Add them to a dashboard
4. Create a new content pack from the current revision (include both the modified dashboard and the stream)
5. Uninstall the content pack, then reinstall.
6. Go on the dashboard and see the problem.

Context

I want to create a combined graph of mean and max latency for requests.

Your Environment

• Graylog Version: 3.0 or 3.0.0-1, taken from docker hub
• Elasticsearch Version: 5.6.15
• MongoDB Version: 3.6.8
• Operating System: Debian, kernel 4.19.0.2, graylog is running in docker
• Browser version: Firefox 65.0.1

[Tastyep]

Bumping this to add some details.
The problem occurs when exporting the content pack, the stacked chart as an empty series field = [],
by modifying manually the JSON I managed to make it work. Correctly exporting it should fix this issue.