[Bug][Chart] The series sum aggregates do not work in Vue 2

[vveesseelliinnaa]

Describe the bug The series aggregates set to sum are not calculated properly in Vue 2 app, leading to the following warning as well:

To Reproduce Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/elastic-jasper-2pthdl

Expected behavior The calculated sum should look like in Vue 3 - https://stackblitz.com/edit/catwdq-occjfv?file=src%2Fmain.vue

Additional context Reported in Ticket ID: 1628989