load fonts on style set change

modules/fonts.js

@@ -0,0 +1,63 @@
+// helper finction to work with fonts
+
+function loadUsedFonts() {
+  const fontsInUse = getFontsList(svg);
+  const fontsToLoad = fontsInUse.filter(font => !fonts.includes(font));
+  if (fontsToLoad) {
+    const url = "https://fonts.googleapis.com/css?family=" + fontsToLoad.join("|");
+    addFonts(url);
+  }
+}
+
+function getFontsList(svg) {
+  const fontsInUse = [];
+
+  svg.selectAll("#labels > g").each(function () {
+    if (!this.hasChildNodes()) return;
+    const font = this.dataset.font;
+    if (font) fontsInUse.push(font);
+  });
+  if (legend.node().hasChildNodes()) fontsInUse.push(legend.attr("data-font"));
+
+  return [...new Set(fontsInUse)];
+}
+
+// code from Kaiido's answer https://stackoverflow.com/questions/42402584/how-to-use-google-fonts-in-canvas-when-drawing-dom-objects-in-svg
+function GFontToDataURI(url) {
+  if (!url) return Promise.resolve();
+  return fetch(url) // first fecth the embed stylesheet page
+    .then(resp => resp.text()) // we only need the text of it
+    .then(text => {
+      let s = document.createElement("style");
+      s.innerHTML = text;
+      document.head.appendChild(s);
+      const styleSheet = Array.prototype.filter.call(document.styleSheets, sS => sS.ownerNode === s)[0];
+
+      const FontRule = rule => {
+        const src = rule.style.getPropertyValue("src");
+        const url = src ? src.split("url(")[1].split(")")[0] : "";
+        return {rule, src, url: url.substring(url.length - 1, 1)};
+      };
+      const fontProms = [];
+
+      for (const r of styleSheet.cssRules) {
+        let fR = FontRule(r);
+        if (!fR.url) continue;
+
+        fontProms.push(
+          fetch(fR.url) // fetch the actual font-file (.woff)
+            .then(resp => resp.blob())
+            .then(blob => {
+              return new Promise(resolve => {
+                let f = new FileReader();
+                f.onload = e => resolve(f.result);
+                f.readAsDataURL(blob);
+              });
+            })
+            .then(dataURL => fR.rule.cssText.replace(fR.url, dataURL))
+        );
+      }
+      document.head.removeChild(s); // clean up
+      return Promise.all(fontProms); // wait for all this has been done
+    });
+}