Add previous years values on slider variables.

src/routes/__layout.svelte

@@ -864,7 +864,7 @@
             max: 100 * slider.stepValue,
             min: 0,
             name: slider.name,
-            period: $year.toString(),
+            period: $year.toString(), // Previous years are added later.
             situationIndex: previousSituationsPopulationCount,
           }
           axisBySituationIndex = {
@@ -901,6 +901,22 @@
           situationIndex1.localeCompare(situationIndex2),
         )
         .map(([, axis]) => axis)
+        .reduce((parallelAxes, axis) => {
+          // Add previous years of each axis.
+          const year = parseInt(axis.period)
+          parallelAxes.push(
+            {
+              ...axis,
+              period: (year - 2).toString(),
+            },
+            {
+              ...axis,
+              period: (year - 1).toString(),
+            },
+            axis,
+          )
+          return parallelAxes
+        }, []) as Axis[]
       $axes = parallelAxes.length === 0 ? [] : [parallelAxes]
 
       let updatedVectorLength = 1