fix: use average instead of sum as default aggregation for time series plot

The initial plot type for time filter animations defaults to
AGGREGATION_TYPES.sum, causing misleading y-axis values (e.g.
earthquake magnitudes showing as 2000/4000/6000 instead of actual
values). All other aggregation defaults in the codebase use average.

Fixes #3122

Signed-off-by: pierreeurope <pierre.europe@pm.me>

Author: pierreeurope

src/utils/src/plot.ts

@@ -698,6 +698,6 @@ export function getDefaultPlotType(filter, datasets) {
     interval,
     defaultTimeFormat,
     type: PLOT_TYPES.histogram,
-    aggregation: AGGREGATION_TYPES.sum
+    aggregation: AGGREGATION_TYPES.average
   };
 }