Diverging stacked bar chart with exact SVG geometry

# Diverging stacked bar chart with exact geometry `input.json`: `categories` (list of names) and `series` (each `{name, color, values}`, one value per category; `null` means 0). Write `solve.py` (Python 3 standard library only); `python solve.py input.json answer.json` must write `answer.json` and a static `chart.svg` in the working directory. Fresh inputs are generated. ## Rules 1. Canvas 720x360. Plot area x0=60, y0=20, width 600, height 300. 2. For each category, `pos` = sum of positive values and `neg` = sum of negative values (null and 0 add nothing). `totals` = [[pos, neg], ...] in category order. 3. Raw domain: lo = min(0, min neg), hi = max(0, max pos). 4. Nice ticks: raw = (hi - lo)/5 (use 1 if hi == lo); mag = 10^floor(log10(raw)); f = raw/mag; m = 1 if f<=1, 2 if f<=2, 2.5 if f<=2.5, 5 if f<=5, otherwise 10; step = m*mag. Axis min = floor(lo/step)*step, axis max = ceil(hi/step)*step (if they are equal, add one step). Round the quotient to 9 decimals before floor/ceil so float noise doesn't add a step. Ticks run from axis min to axis max in steps (each rounded to 10 decimals). `domain` = [axis min, axis max]. 5. Tick label: the smallest d in 0..6 such that step rounded to d decimals equals step; format each tick with d decimals; a label equal to zero prints as `0`. 6. y(v) = y0 + 300*(axis max - v)/(axis max - axis min). 7. Band = 600/number of categories. Bar x = x0 + i*band + 0.15*band; width = 0.7*band. 8. Stacking: series in the given order. Positive values stack **upward from 0** on the positive stack; negative values stack **downward from 0** on a separate negative stack. A value of 0 or null draws no bar. 9. A bar from value bot to top has y = y(top) and height = y(bot) - y(top). Round x, y, width and height to 2 decimals. 10. Bar id `bar-<category index>-<series index>` (0-based), with fill = the series color. ## Outputs - `answer.json`: `{"domain", "ticks", "tick_labels", "totals", "bars": [{"id","x","y","width","height","fill"}, ...]}`, with bars in category order and then series order. - `chart.svg`: root `<svg>`, no scripts, external resources or animation. It contains `<g id="bars">` holding one `<rect id="bar-i-j" x y width height fill>` per bar, with numeric attributes equal to the JSON values (within 0.01). For tick k: `<line id="tick-k">` at y(tick) and `<text id="tick-label-k">` holding the label. Also `<line id="zero-line">` at y(0). ## Scoring Hidden checks (partial credit; all must pass): `stack_totals` (1e-6), `nice_ticks_domain` (exact labels), `bar_geometry_json` (0.011), `chart_svg_static` (parses as static SVG with the required ids and tags), `chart_svg_geometry` (rect attributes and tick-label texts read from the SVG itself), `fresh_run_1`, `fresh_run_2` (domain, labels, bars and SVG geometry on new data). 30-second limit per run.