reports: add ability to group reports by column.

Add ability to group reports by various columns. For example, you can see
the posts by the top 10 uploaders over time, or posts grouped by rating
over time.

app/javascript/src/javascripts/time_series_component.js

@@ -11,7 +11,7 @@ export default class TimeSeriesComponent {
 
     this.options = {
       dataset: {
-        dimensions: ["date", ...columns],
+        dimensions: ["date", ...this.columns],
         source: data,
       },
       tooltip: {
@@ -46,14 +46,15 @@ export default class TimeSeriesComponent {
         containLabel: true
       },
       legend: {
-        data: columns.map(startCase),
+        data: this.columns.map(startCase),
       },
       xAxis: { type: "time" },
-      yAxis: columns.map(name => ({ type: "value" })),
-      series: columns.map(name => ({
+      yAxis: this.columns.map(name => ({ type: "value" })),
+      series: this.columns.map(name => ({
         name: startCase(name),
         type: "line",
         areaStyle: {},
+        stack: "all",
         emphasis: {
           focus: "series"
         },