Feature request: stacked grouped mixed chart

[beliaev-maksim]

Business requirement specification based on #28181

Request

We need a bar chart that supports both grouping and stacking simultaneously. The X-axis should handle arbitrary data or time series. Additionally, the chart must support a mixed format (e.g., bar + line overlay) and include trend lines.

Example Use Case:

We currently track the ratio of User Stories delivered in Sprint N to Bugs opened in Sprint N (split by P1 and P2/P3). An efficiency line (#User Stories / #Bugs Open) helps visualize team performance, while a trend line for P1 bugs highlights patterns over time.

This is critical for maintaining product quality as we scale. As technical debt grows, bug influx can impact efficiency. Tracking these metrics ensures we maintain high standards while keeping the product competitive.

Google Sheet implementation

Example of implementation using Google Sheet: https://docs.google.com/spreadsheets/d/1G3rntcOYkf9Uwz4yBHDFOsfgV87RT0UMh15HbxTXlh4/edit?usp=sharing

The chart groups User Stories (blue bars) and Bugs (red + yellow bars) by Agile sprints. At the same time, Bugs are stacked to show priority levels (red for Priority 1, yellow for Priority 2/3).

The green line represents efficiency across sprints, while the light red line is an automatically calculated trend line for Priority 1 Bugs.