Sparkline: Fixes issue with sparkline that sent in custom fillColor instead of fillOpacity (#29825)

* Sparkline: Fixes issue with sparkline that sent in custom fillColor instead of fillOpacity

* Updated

packages/grafana-ui/src/components/uPlot/config/UPlotSeriesBuilder.ts

@@ -73,14 +73,19 @@ export class UPlotSeriesBuilder extends PlotConfigBuilder<SeriesProps, Series> {
 
     let fillConfig: any | undefined;
     let fillOpacityNumber = fillOpacity ?? 0;
-    if (fillColor && fillOpacityNumber !== 0) {
+
+    if (fillColor) {
       fillConfig = {
-        fill: tinycolor(fillColor)
-          .setAlpha(fillOpacityNumber / 100)
-          .toRgbString(),
+        fill: fillColor,
       };
     }
 
+    if (fillOpacityNumber !== 0) {
+      fillConfig.fill = tinycolor(fillColor ?? lineColor)
+        .setAlpha(fillOpacityNumber / 100)
+        .toRgbString();
+    }
+
     return {
       scale: scaleKey,
       spanGaps: spanNulls,