Scroll the runs table, pick a range, and choose what a comparison plots against

docs/code/api.md

@@ -109,7 +109,7 @@ published to. Flows own the namespace; everything else is a client of it.
 | `GET` | `/runs/{id}` | one run in full: params, result, per-node record, artifacts |
 | `POST` | `/runs/{id}/cancel` | stop it |
 | `GET` | `/runs/{id}/metrics?name=&stride=` | one metric's series, in step order; every series of the run without `name` |
-| `GET` | `/runs/series/compare?ids=a,b,c&metric=` | that metric across several runs |
+| `GET` | `/runs/series/compare?ids=a,b,c&metric=&x=` | that metric across several runs. `x` is what to plot against: nothing or `step`, `time` (seconds since each run's own first reading), or another metric's name, joined on the step the two share |
 
 Submitting answers immediately with a `queued` run. Wrong parameters — an
 undeclared name, a value of the wrong type — come back as a 422 naming the

docs/concepts/runs.md

@@ -295,9 +295,16 @@ flow, by status, or down to one sweep. A sweep is worth filtering to — the
 table then draws a column per parameter that actually varied, which is what
 makes fifty runs of one flow readable.
 
-Tick two or more and their curves go side by side. That comparison is the
+Tick two or more and their curves go side by side — shift-click to take a
+range, or the header box to take everything on screen. That comparison is the
 address, so a link to it is a link someone else can open.
 
+The curves are drawn against the step by default. They can also be drawn
+against elapsed seconds, which answers "which one got there sooner" and is
+measured from each run's own first reading so that runs started hours apart
+still lie on top of each other; or against another metric of the same runs —
+an epoch, or samples seen — joined on the step the two share.
+
 One run in full is params, the per-node record with its logs and traceback,
 the artifacts it made, its metrics and its result.