I am reminded of when I worked on a financial analytics app and we constantly had to remove the Zimbabwe stocks. They'd all gone up 1000's of percent and ruined all the other data on the chart (appearance-wise). Why? As the Zimbabwe currency lost value and became worthless, the stocks denominated in that currency held their real value. I would expect to see some version of this play out in the stock market today.
I have near zero knowledge of finance infrastructure, but shouldn't stock price be normalized by the exchange rate in which said stock is denominated? Say index everything in USD or whatever currency your user prefers. Otherwise, even if you remove Zimbabwe stocks from your charts, any analytics will be fundamentally flawed.
You are right that most carefully presented charts will correct for exchange rate differences over time. However, a database of stocks will often keep performance over different time periods in the native currency as a convenience. For example, this is usually good enough for most screening applications, and on-the-fly charting apps.
