Allow non-numeric axes in XY chart

**Why is this needed**: Currently, there is no easy way to create a scatter plot where the X or Y axis is not numeric. For example, if I want to plot several samples of `Energy Consumption (kWh)` by `Country` (e.g. US, UK, JP), the Grafana XY chart complains because the `Country` field is not numerical. <img src="https://github.com/user-attachments/assets/7cf2dbc3-bf3c-45cf-966a-2f9c11ab8329" width="500" /> **What would you like to be added**: The Grafana XY chart should have support for non-numeric data (i.e. categories) as X or Y axis data. **Who is this feature for?** This is such a common use case that affects the entire Grafana user base.