Show the delta of the selected area in a tooltip, as Google does for stocks

[hermesramos]

Summary

Add a feature to display the percentage change (delta) for a selected area on charts, similar to how Google displays stock data. When users select a portion of the chart, a tooltip should appear, showing the percentage change over that time period. This feature will enhance the user experience by providing more intuitive insights into trends and changes.

For reference, see the example below:

API Changes

Introduce a method to calculate the delta for the selected range.

Update the chart API to include an option for showing/hiding the delta tooltip.

Intended Use Case

This feature will allow users to perform quick simulations and gain insights directly on the charts by seeing the percentage change over a selected period. It will be especially useful for financial analysis, helping users make more informed decisions based on historical trends.