Enhancement - making the tooltip more readable (https://bugs.gnucash.org/show_bug.cgi?id=799799)

https://bugs.gnucash.org/show_bug.cgi?id=799799

A small change for making the tooltip more readable as it can get very messy with bigger data-sets. (see the screenshot)

>> Screenshots based on: GC Sample Database:
https://github.com/PRG-112/gnucash-sample-db-generator

gnucash/report/html-chart.scm

@@ -374,7 +374,7 @@ function tooltipLabel(tooltipItem,data) {
   var label = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];
   switch (typeof(label)) {
     case 'number':
-      return datasetLabel + ': ' + numformat(label);
+      return '  ' + datasetLabel + ' :   ' + numformat(label);
     default:
       return '';
   }