No, I don't think there's any plugin for collectd graphs.
I think you're right to stick to the Discover tab until you find you're getting the metrics you're looking for. Then go to Visualize to pick what kind of charts you want, and finally Dashboard to put those visualizations together.
When you go to Discover, I would enter each of your search criteria one at a time and watch the count of hits in the top right corner of the chart. If you add a search criteria and your hits go to 0 you know there's a problem.
And I think you might be doing something wrong on your last "value:??". The search is used to filter out the documents you want (not the data you want from those documents). Try leaving that off.
Try instead to find the "value" field in the left pane on the discover tab and click [Add] on that. That should show you the data under the bar graph.
Save that search and then go to Visualize and select from that saved search.
Maybe a line chart would be good? On the Y-Axis change it from Count to Sum and then the Field selection list appears below it. Here you might pick "value" if that field has the memory used value.
The in the buckets pick X-Axis,
Aggregation: Date Histogram
Field: @timestamp (if you have that)
and click the green "Go" arrow
Does that show you anything like what you're looking for?