simplify the availability test (#2389)

Author: Fil

test/plots/availability.ts

@@ -3,23 +3,20 @@ import * as d3 from "d3";
 
 export async function availability() {
   const data = await d3.csv<any>("data/availability.csv", d3.autoType);
-  const sum = (d) => (d.length ? d3.sum(d) : NaN); // force gaps
   return Plot.plot({
     height: 180,
     marks: [
       Plot.areaY(data, {
         x: "date",
         y: "value",
         interval: "day",
-        reduce: sum,
         curve: "step",
         fill: "#f2f2fe"
       }),
       Plot.lineY(data, {
         x: "date",
         y: "value",
         interval: "day",
-        reduce: sum,
         curve: "step"
       }),
       Plot.ruleY([0])