Make plotting functions composable

Use any approach that allows the caller to compose the plots together without side-effects. This should be done without requiring calling code to add boilerplate. Typically one passes the axes to the plotting function. If no axes are passed, then it just creates a new figure or something like that.

Look for plotting functions in the codebase, likely in modules related to visualization or reporting. Understand how figures and axes are currently managed. The goal is to modify these functions to accept optional axes parameters, defaulting to creating a new figure if none provided, ensuring composability without side effects. Check for existing tests to understand the expected behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.