selectFilter((I, V) => index, {key, ...options})

ref. #190 (comment)
ref. #138

Author: Fil

src/index.js

@@ -19,5 +19,5 @@ export {TickX, TickY, tickX, tickY} from "./marks/tick.js";
 export {bin, binX, binY, binR} from "./transforms/bin.js";
 export {group, groupX, groupY} from "./transforms/group.js";
 export {movingAverageX, movingAverageY} from "./transforms/movingAverage.js";
-export {selectFirst, selectLast, selectMaxX, selectMaxY, selectMinX, selectMinY} from "./transforms/select.js";
+export {selectFilter, selectFirst, selectLast, selectMaxX, selectMaxY, selectMinX, selectMinY} from "./transforms/select.js";
 export {stackX, stackX1, stackX2, stackXMid, stackY, stackY1, stackY2, stackYMid} from "./transforms/stack.js";

src/transforms/select.js

@@ -25,6 +25,10 @@ export function selectMaxY(options = {}) {
   return {...options, transform: select(max, "y", options)};
 }
 
+export function selectFilter(f, {key, ...options} = {}) {
+  return {...options, transform: select(f, "key", {key})};
+}
+
 function* first(I) {
   yield I[0];
 }