Describe the bug
The current extra value displayed on the legend renders by default the last value on the series. This feature is only interesting and useful when rendering charts with a continuous scale along the x. In a categorical scale, the last value doesn't have any special meaning.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/ecstatic-colden-bgzgj?file=/src/App.tsx
2. The legend is showing the value related to YOW column

Expected behaviour
For ordinal scales along the x axis, the legend extra value should be not displayed until it can be customized to display something else than the last value of the series

Screenshots

Version (please complete the following information):

• Elastic Charts: all