XY Chart: Add "Group by" / "Split by" field option for automatic multi-series with distinct colors

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: The XY Chart cannot automatically split data into multiple colored series based on a categorical field. This makes it unsuitable for common analytical visualizations like cohort analysis, A/B test comparisons, and multi-entity trend analysis. For example, with data structured as (week_idx, cohort, ltv), there's no way to get separate colored lines per cohort. The existing "Color Field" option only supports continuous numeric gradients, not categorical grouping. Current workarounds all have significant drawbacks: * Pivoting data in SQL to wide format doesn't scale and requires manual maintenance as categories change * Transformations like "Partition by values" are buggy and don't integrate well with XY Chart * Third-party plugins (Business Charts, Plotly) require custom JavaScript **What would you like to be added**: A "Group by" or "Split by" field option in XY Chart that accepts a categorical field and automatically: * Creates one series per unique value * Assigns distinct colors from the selected color scheme * Populates the legend with each group's name **Who is this feature for?** Data analysts and engineers building analytical dashboards with non-time-series data: cohort analysis, scatter plots by category, trend comparisons across dimensions, and any visualization requiring categorical color grouping on an XY plane.