Q:

Can not add custom field to Y-Axis Average aggregation for visualization

When I try to add Bar Chart visualization, I cannot select any custom field for my messages for Average aggregation (like: "my_custom_field").  Messages are logged and contain this field.  How to do this? 

A:

I don't understand why, but when I create visualization via Visualizations tab/Add new/Vertical bar chart, fields are not present. 
But, I can make visualization by:

[OPTIONAL, probably didn't help] Go to Management/Index Patterns and click Refresh image button in right top corner.  This will create mapping types for fields that are new since index creation (this was in my case).  Such field can be recognized when before its name there is "?",  means no type (e.g. # means field has type, like on the image below). 
Go to Discover tab and select your field on the left, than click Visualize -> that will allow to select Average aggregate with your field. 

This is very inconvenient, but works.