fix(cli): clear previous graph before plotting in realtime mode

[zhengshui]

Summary

This PR fixes issue #44 by improving the realtime graph clearing behavior.

Changes

• Added lastGraphLines variable to track the previous graph height
• Added clearPreviousGraph() function that clears only the previous graph lines instead of clearing the entire screen using ANSI escape sequences
• This reduces screen flicker in realtime mode

How it works

1. After each graph is plotted, we record the number of lines in lastGraphLines
2. Before plotting the next graph, we use ANSI escape sequences to:
   • Move cursor up by lastGraphLines
   • Clear each previous line
   • Move cursor back to start position

Verification

go test ./...
# Build and test realtime mode
echo -e "1\n2\n3\n4\n5\n6\n7\n8\n9\n10" | go run ./cmd/asciigraph -r

All tests pass.

Closes #44