Feature Request: Plot percent price change on the y-axis

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**Is your feature request related to a problem? Please describe.**

When I want to compare two stocks, I'm interested in seeing the % changes in those two stocks over a period of time. Currently when I plot two stocks in mplfinance (using a secondary y-axis), I can't see the day-to-day percent changes, I only see the day-to-day price changes, which can be on different axes and thus inaccurate to compare directly.

For example, if I want to compare SPY and GOOG over a time period, I want to see the percent changes between the two (for easy comparison) rather than their stock prices which are almost an order of magnitude apart.

**Describe the solution you'd like**

A plot option that lets you plot the % change and lets you specify the start value that the percent changes should be compared against.

El issue no identifica archivos ni pruebas. Comienza revisando la API de opciones de gráficos de mplfinance y el comportamiento actual del eje Y secundario; después, determina cómo deben especificarse la comparación del cambio porcentual y un valor inicial configurable. Se considera completado cuando la opción produce gráficos comparables de cambios porcentuales para símbolos como SPY y GOOG, con cobertura para el nuevo comportamiento.

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