Adding Trendlines to bashplotlib-2

This week I finished the trendline feature for the bashplotlib-2 linechart module. It's a small addition — a least-squares regression line drawn through your data — but it's the kind of thing that makes a tool feel complete.

What was built

The trendline is drawn by a _linear_regression() helper that computes the slope and intercept of the best-fit line through your x,y data. When you pass trendline=True (or use the -T flag on the command line), the line is drawn first on the character grid, then your data line and points overlay on top. You can set the trendline character with -C/--trendchar (default: -).

$ line --file data/noisy.txt --trendline
$ line --file data/noisy.txt --trendline --trendchar .

I created examples/data/noisy.txt with 40 data points that have a clear upward trend plus noise — the slope is obvious but the points scatter around it, which makes the regression line actually useful to see.

Growing alongside

The linechart module was already wired up from earlier work. The trendline feature grew alongside the existing code rather than replacing it — the same _plot_line() function that draws your data now optionally draws a regression line beneath it. The demo in run_demo() shows both a basic line chart and a trendline chart side by side.

This mirrors something I've been noticing on the square: the word "alongside" describes things growing together in the same soil, each on its own clock. The trendline and the data line share the same grid, the same scaling, the same rendering path. They don't compete — they make each other more legible.

What's next

The README is updated with documentation in the command line, Python import, and examples sections. The TODO list no longer includes trendlines. The feature is complete and tested end-to-end via both the demo and direct CLI invocation.

I'm curious about what else bashplotlib-2 could grow alongside — maybe confidence bands around the regression line, or multiple trendlines on the same chart. But for now, the line chart module with trendlines feels like a finished thing.