draw labels marking the centroids of category value clusters (#809)

* Connect mouse over events to reducer actions

* Change Styling on hover

* Rename reducer actions to be more descriptive

* Reorder reducer in cascade

* Create centroid calculation util

* Whitespace

* Typo fix, use correct action

* Create centroid calc util

* Create centroid svg setup

* Refactor existing svg layer to toolSVG

* Change calcCentroid signature and centroidXY to match mapPointToScreen

* Add id and styling

* Run prettier

* Set z-index to 999

* Draw the label

* Introduce the centroid SVG, refactor code to allow both SVG layers

* Add text label and compute radius based on population

* Implement optional chaining

* Update font family

* Optimize calcMeanCentroid()

* Create and utilize calcMedianCentroid()

* Remove mass circle from label

* Remove styling change on hover

* Remove reducer action logs

* Prettier

* Swap out binds for arrow functions

* Style text

* switch from selectAll() to select()

* Reflect centroid container's purpose in id

* Remove mass from the output

* Swap to obj

* Add finite check

* Don't draw centroid if no finite values

* Fix finite check

* Remove log

* Toggle label coloring based on colorBy state

* Pass cursor events through centroid svg

client/src/util/centroid.js

@@ -0,0 +1,58 @@
+import quantile from "./quantile";
+
+/*
+  Centroid coordinate calculation
+*/
+const calcMeanCentroid = (world, annoName, annoValue, layoutDimNames) => {
+  const centroid = { x: 0, y: 0, size: 0 };
+  const annoArray = world.obsAnnotations.col(annoName).asArray();
+  const layoutXArray = world.obsLayout.col(layoutDimNames[0]).asArray();
+  const layoutYArray = world.obsLayout.col(layoutDimNames[1]).asArray();
+
+  for (let i = 0, len = annoArray.length; i < len; i += 1) {
+    if (annoArray[i] === annoValue) {
+      centroid.x += layoutXArray[i];
+      centroid.y += layoutYArray[i];
+      centroid.size += 1;
+    }
+  }
+
+  if (centroid[2] !== 0) {
+    centroid.x /= centroid.size;
+    centroid.y /= centroid.size;
+  }
+
+  return [centroid.x, centroid.y];
+};
+
+const calcMedianCentroid = (world, annoName, annoValue, layoutDimNames) => {
+  const centroidX = [];
+  const centroidY = [];
+  let hasFinite = false;
+
+  const annoArray = world.obsAnnotations.col(annoName).asArray();
+  const layoutXArray = world.obsLayout.col(layoutDimNames[0]).asArray();
+  const layoutYArray = world.obsLayout.col(layoutDimNames[1]).asArray();
+
+  for (let i = 0, len = annoArray.length; i < len; i += 1) {
+    if (annoArray[i] === annoValue) {
+      hasFinite =
+        Number.isFinite(layoutXArray[i]) || Number.isFinite(layoutYArray[i])
+          ? true
+          : hasFinite;
+      centroidX.push(layoutXArray[i]);
+      centroidY.push(layoutYArray[i]);
+    }
+  }
+
+  if (hasFinite) {
+    const medianX = quantile([0.5], Float64Array.from(centroidX));
+    const medianY = quantile([0.5], Float64Array.from(centroidY));
+
+    return [medianX, medianY];
+  }
+
+  return null;
+};
+
+export default calcMedianCentroid;