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/reducers/centroidLabel.js

@@ -0,0 +1,44 @@
+import calcCentroid from "../util/centroid";
+
+const initialState = {
+  metadataField: "",
+  categoryIndex: -1,
+  categoryField: "",
+  centroidXY: [-1, -1]
+};
+
+const CentroidLabel = (state = initialState, action, sharedNextState) => {
+  const { categoricalSelection, world, layoutChoice } = sharedNextState;
+  const { metadataField, categoryIndex } = action;
+  const categoryField =
+    categoricalSelection?.[metadataField]?.categoryValues[categoryIndex];
+  switch (action.type) {
+    case "category value mouse hover start":
+      return {
+        ...state,
+        metadataField,
+        categoryIndex,
+        categoryField,
+        centroidXY: calcCentroid(
+          world,
+          metadataField,
+          categoryField,
+          layoutChoice.currentDimNames
+        )
+      };
+
+    case "category value mouse hover end":
+      if (
+        metadataField === state.metadataField &&
+        categoryIndex === state.categoryIndex
+      ) {
+        return initialState;
+      }
+      return state;
+
+    default:
+      return state;
+  }
+};
+
+export default CentroidLabel;