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

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.

Issue ではファイルもテストも特定されていません。まず mplfinance の plot-option API と現在の secondary-y-axis の動作を確認し、そのうえで、パーセント変化の比較と設定可能な開始値をどのように指定すべきかを決定してください。SPY や GOOG などのシンボルについて、オプションによって比較可能なパーセント変化のプロットが生成され、新しい動作をカバーできていれば完了です。