src/components/HistoryChart/HistoryChart.tsx

@@ -70,6 +70,35 @@ function mergeBuckets(
   );
 }
 
+function attachComparison(
+  points: ChartDataPoint[],
+  key: "weekly" | "monthly" | "yearly" | "fyly"
+): ChartDataPoint[] {
+  const getCompareIndex = (i: number) => {
+    if (key === "weekly") return i - 4;
+    if (key === "monthly") return i - 12;
+    if (key === "yearly") return i - 1;
+    if (key === "fyly") return i - 1;
+    return -1;
+  };
+
+  return points.map((p, i) => {
+    const ci = getCompareIndex(i);
+
+    return {
+      ...p,
+      compare:
+        ci >= 0 && points[ci]
+          ? {
+              id: points[ci].id,
+              label: points[ci].label,
+              amount: points[ci].amount
+            }
+          : undefined
+    };
+  });
+}
+
 function buildChartData(
   reportData: HistoryChartProps["reportData"],
   key: "weekly" | "monthly" | "yearly" | "fyly",
@@ -86,17 +115,7 @@ function buildChartData(
   }));
 
   if (comparison) {
-    points = points.map((p, i) => ({
-      ...p,
-      compare:
-        i > 0
-          ? {
-              id: points[i - 1].id,
-              label: points[i - 1].label,
-              amount: points[i - 1].amount
-            }
-          : undefined
-    }));
+    points = attachComparison(points, key);
   }
 
   return points;