move graph scales, still hardcoded, into globals

src/globals.js

@@ -41,3 +41,21 @@ export const API = {
 export const accentFont = "Georgia,Times,Times New Roman,serif";
 export const maxParagraphWidth = 600;
 export const maxControlsWidth = 800;
+
+export const graphMargin = {top: 20, right: 10, bottom: 30, left: 40};
+export const graphWidth = 960;
+export const graphHeight = 500;
+
+export const graphXScale = d3.scaleLinear()
+  .domain([0, 1]) /* while this is the default for d3, our data is normalized so better to be explicit */
+  .range([
+    0 + graphMargin.left,
+    graphWidth - graphMargin.right
+  ]);
+
+export const graphYScale = d3.scaleLinear()
+  .domain([0, 1]) /* while this is the default for d3, our data is normalized so better to be explicit */
+  .range([
+    graphHeight - graphMargin.bottom,
+    0 + graphMargin.top
+  ]);