Tweak style for scatterplot trendline

Use different color for the trendline and bring to front

ee/app/assets/javascripts/analytics/shared/constants.js

@@ -4,15 +4,12 @@ export const dateFormats = {
   defaultDateTime: 'mmm d, yyyy h:MMtt',
 };
 
-/**
- * #1f78d1 --> $blue-500 (see variables.scss)
- */
-export const scatterChartLineColor = '#1f78d1';
-
 export const scatterChartLineProps = {
   default: {
     type: 'line',
     showSymbol: false,
-    lineStyle: { color: scatterChartLineColor },
+    // By default zlevel is 2 for all series types.
+    // By increasing the zlevel to 3 we make sure that the trendline gets drawn in front of the dots in the chart.
+    zlevel: 3,
   },
 };