[Chart Renderer]

Chart Renderer

Abstract

backend/services/chart_renderer.py is a headless plotting façade. It forces matplotlib into Agg, draws dark-theme charts for desk thumbnails, and returns base64-encoded PNG strings suitable for JSON transport. Callers are the preview and backtest routes; the module has no Flask dependency.

Conceptual model

Interface surface

render_line_chart

Empty / all-null values → _render_empty_chart placeholder.

render_ohlcv_chart

Candlestick + volume subplot pair from dict keys open, high, low, close, volume. Green/red body colors (#4CAF50 / #F44336). Shared x-axis.

render_equity_curve

Single series with profit/loss fill relative to zero baseline and a dashed reference at the first equity point. Title fixed "Equity Curve".

Return type

All public renderers → str (base64 PNG without data-URI prefix). Clients typically use:

data:image/png;base64,{chart}

Internals

Invariants and edge cases

1. Not interactive — no GUI backend; safe for servers.
2. NaNs break lines at gaps (numpy nan).
3. Candle body height uses + 1e-9 to keep zero-range bodies visible.
4. Thread safety: matplotlib global state is historically process-local; multi-threaded gunicorn workers should avoid concurrent pyplot use on one worker or serialize renders.
5. Exceptions in callers often become HTTP RENDER_ERROR or empty equity chart strings.

Worked example

from backend.services.chart_renderer import render_line_chart

b64 = render_line_chart([1.0, 1.2, 1.1, 1.4], title="demo", color="#A3E635")
assert isinstance(b64, str) and len(b64) > 100

Failure modes

See also

• Preview endpoints
• Backtest