The Labstack Currency API is a simple, no-frills currency conversion API that supports over 150 currencies and is fast and accurate. To use the API, developers must first sign up for a free RapidAPI developer account, navigate to the API console, subscribe to the API, and test the API endpoints. The API has a freemium subscription model with a basic plan offering 400 API calls per month for free. The API supports two main endpoints: "GET Convert" for currency conversion between two currencies and "GET List" for retrieving all supported currencies. To build a custom currency conversion chart in Python, developers must first import the necessary libraries, define functions for drawing the grid and adding currency rates, and then call the API to retrieve the conversion rates and populate the chart. The resulting SVG file can be opened in any browser or used as needed in various applications.