How to find problematic causes in jira with custom plugins ?

[Reinis]

Hi. 
I have been trying to figure out why pie chart doesn't show our custom field, that we are adding via custom plugin. Its a multiUser selector field, but with some added logic. And when selecting it in pie chart diagram, it doesn't show all data. It kinda works the opposite way. 
If I select my field, it shows results for none. So where our field is empty.

I can see that jira fires a rest call to: jira/rest/gadget/1.0/statistics?jql=***
And it returns only the none value, not like it does for jiras built in field type.

I have two of those pie charts on, so i see two rest calls, and one is configured to use default jira multiUserPicker and one is our custom picker. 

And I can see that it returns wrong json from that rest call. But I cannot find anywhere any documentation, about that rest api. Why It would return incorrect results, what could be the problem, or any kind of information. 

What is the procedure to find out what jira does on that rest api and why it cannot find my field ? Because of course there are options to implement custom filters, that manually override that rest call, and use different values that I can get from DB or any other way, but it doesn't seem the right way to do it. 

[Alexander Pappert]

is it the jira pie chart or from another addon?
you need to check if this supports your custom field from the other plugin

what type of custom field is it?

name, number, something that can't be displayed in pie charts

[Reinis]

@Alexander Pappert 
It is the jira pie chart. 
And its of type User Picker (multiple users), basically the same but rewritten to meet our needs. (I did not write this plugin, just trying to figure out why pie chart wont show results for it)

custom field of default type - User Picker (multiple users) - WORKS in pie chart
custom field of our type - User picker (multiple users) - Doesnt work in pie chart.


I think if I could find the source that gets called on that rest api call, I could figure out whats wrong. There could be something wrong with our plugin (That is custom User picker), but I dont know what, and I dont know where to start debugging. Thats way I'm trying to find what happens behind that "jira/rest/gadget/1.0/statistics?jql=***" call. But there is no documentation about it nor about pie chart, or anything else..
There are many packages inside atlasian sdk, and to find the one thats responsible blindly is impossible.