If you are using ElasticSearch query, make sure your resulting data is what you expected. The easiest way to view it is by using "networking" tab in the browser debugging tools (e.g. F12). Modify the graph slightly so that it makes a search request, and view the response from the server. Another approach is to use Kibana Dev Tools tab - place the index name into the first line: GET <INDEX_NAME>/_search, and add your query as the following lines (just the value of the "query" field)

If you need to share your graph with someone, you may want to copy the raw data response to, possibly with a .json extension, use the [raw] button, and use that url directly in your graph.

To restrict Vega from using non-ES data sources, add vega.enableExternalUrls: false to your kibana.yml file.