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.
