Support hardware counters in plotting scripts (#984)

scripts/README.md

@@ -7,6 +7,14 @@ hyperfine 'sleep 0.020' 'sleep 0.021' 'sleep 0.022' --export-json sleep.json
 uv run plot_whisker.py sleep.json
 ```
 
+All plotting scripts accept `--metric` to select a metric. Hardware counters
+(`cpu_cycles`, `instructions`, `cache_references`, `cache_misses`, and `branch_misses`)
+are plotted as unscaled counts when available in the JSON export. For example:
+
+```bash
+uv run plot_histogram.py benchmark.json --metric instructions
+```
+
 ### Prerequisites
 
 Install [`uv`](https://docs.astral.sh/uv/getting-started/installation/) and run the