Add nvbench-compare-robust --plot-output/--plot-along-output options

docs/nvbench_compare_robust.md

@@ -64,14 +64,25 @@ nvbench-compare-robust --display explain reference.json compare.json
 nvbench-compare-legacy reference.json compare.json
 ```
 
-Plot the comparison summary, or plot timings along a positive numeric axis. Add
+Plot the comparison summary, or plot timings along a positive numeric axis. By
+default, plotting uses Matplotlib's interactive `plt.show()` behavior. Add
 `--dark` to the summary plot when it should use a dark theme:
 
 ```bash
 nvbench-compare-robust --plot --dark reference.json compare.json
 nvbench-compare-robust --plot-along "Elements{io}" reference.json compare.json
 ```
 
+Save plots to files when running in CI, remote shells, or scripted workflows:
+
+```bash
+nvbench-compare-robust --plot --plot-output compare.png reference.json compare.json
+nvbench-compare-robust \
+  --plot-along "Elements{io}" \
+  --plot-along-output "plots/{benchmark}-device{device}-{axis}.png" \
+  reference.json compare.json
+```
+
 Generate Python code with bulk sample/frequency filenames for every displayed
 row:
 
@@ -646,3 +657,42 @@ fraction: use `--threshold-diff 5` for a 5% threshold.
 
 This option affects table output. It does not change summary counters or the
 data used by `--plot-along`.
+
+### `--plot-output PATH`
+
+Save the summary plot generated by `--plot` to `PATH` and do not call
+`plt.show()`. When this option is omitted, `--plot` keeps the default
+interactive behavior.
+
+For directory comparisons that produce more than one summary plot, a single
+`--plot-output PATH` would overwrite earlier output. In that case the command
+fails instead of writing multiple plots to the same path.
+
+### `--plot-along-output PATH_OR_TEMPLATE`
+
+Save plots generated by `--plot-along` to files and do not call `plt.show()`.
+Because `--plot-along` may produce one plot per benchmark/device pair, this
+option accepts filename templates with these fields:
+
+- `{benchmark}`: benchmark name
+- `{device}`: compare-device id
+- `{axis}`: selected plot axis name
+- `{pair}`: zero-based positional device-pair index
+
+Field values are sanitized before substitution so benchmark or axis names from
+input JSON cannot introduce path separators. Directory structure should be
+written literally in the template, as in `plots/{benchmark}.png`.
+
+For example:
+
+```bash
+nvbench-compare-robust \
+  --plot-along "Elements{io}" \
+  --plot-along-output "plots/{benchmark}-pair{pair}-{axis}.png" \
+  reference.json compare.json
+```
+
+A plain path without template fields is valid only when one plot is generated
+across the whole command. If multiple plot-along figures, including figures
+from different JSON file pairs in directory mode, would write the same path, the
+command fails instead of silently overwriting earlier output.