Make it easy for plugin developers to use sub-tables of any level in their data plugin

[mattab]

The goal of this issue is to make it easier to developers to manage data tables where rows can have nested sub-tables. In particular we may review how Sub-tables are implemented and the API proposed to platform developers. This can be broken down later by client side AngularJS improvements (new angularjs datatable?), and by core analytics platform PHP work. (each technology has its own set of challenges to solve before we can reach a state of beautiful sub tables APIs.)

Context
This feature of displaying data from sub-tables is available in common reports (Referrers > Websites, click on domain and you get all pages that sent you traffic for this website), and in Actions > Pages when you click a directory in the report it then open all pages within this Page URLs path.

The code is a bit convoluted and maybe awkward as it requires to use an idSubtable that sometimes makes things difficult. Instead maybe we could think of a simpler and better way in Piwik to manage tables with nested tables?

Use case

originating from this other comment #6097 (comment) a very nice use case for use of sub-tables of a single datatable.

Note: these sub-tables would be based off a single data table, "grouped by" language only to make visualisation better. (in other words, the server side does not implement idSubtable parameter.)

References
There are several opened issues about sub-tables:

• New data visualisation based on events sub-tables Event reporting: new report to view the second dimension as columns (Pivot by second dimension) #6078
• ux, bug: Make Sub table always loads data for the right row when date=Today #1024, Error in Archivejob: Adding a subtable to the row, but it already has a subtable associated #4528
• better plotting of sub-tables values in graphs Allow to plot custom variable values (or any rows) by searching & plotting - JS Graphs #2786, Allow to plot a subtable as a graph #3165

Tasks

• Identify how this problem was solved before by other tools and data libraries
• Core platform: maybe the idSubtable could be changed to be instead the label of the row we want to get sub-level?
• JS-AngularJS: make it possible to display sub-tables on any datatable (see Use case above)
• TBD

let's discuss in the comments!