--- a/client/src/components/gauge/Waterlevel.vue	Tue May 14 12:10:45 2019 +0200
+++ b/client/src/components/gauge/Waterlevel.vue	Tue May 14 12:24:14 2019 +0200
@@ -762,11 +762,21 @@
       const waterlevelChartDrawer = isNav => {
         return d3
           .lineChunked()
+          .defined(
+            // render only data points that are visible in the current scale
+            d => {
+              let domainLeft = new Date(this.scale.x.domain()[0].getTime());
+              domainLeft.setDate(domainLeft.getDate() - 1);
+              let domainRight = new Date(this.scale.x.domain()[1].getTime());
+              domainRight.setDate(domainRight.getDate() + 1);
+              return d.date > domainLeft && d.date < domainRight;
+            }
+          )
           .x(d => this.scale[isNav ? "x2" : "x"](d.date))
           .y(d => this.scale[isNav ? "y2" : "y"](d.waterlevel))
           .curve(d3.curveLinear)
-          .isNext(this.isNext(900))
-          .pointAttrs({ r: isNav ? 1.7 : 2.2 })
+          .isNext(this.isNext())
+          .pointAttrs({ r: 1.7 })
           .chunk(d => (d.predicted ? "predicted" : "line"))
           .chunkDefinitions({ predicted: {} });
       };
@@ -1159,10 +1169,20 @@
             .attr("height", textBBox.height + tooltipPadding * 2);
         });
     },
-    isNext(seconds) {
-      // helper to check whether points in the chart are "next to each other"
-      // for that they need to be exactly the specified amount of seconds apart.
-      return (prev, current) => current.date - prev.date === seconds * 1000;
+    isNext() {
+      // Check whether points in the chart can be considered "next to each other".
+      // For that they need to be exactly 15 minutes apart (for automatically
+      // imported gauge measurements). If the chart shows more than 15 days then
+      // 1 hour is also valid (for approved gauge measurements).
+      return (prev, current) => {
+        let difference = (current.date - prev.date) / 1000;
+        if (
+          (this.scale.x.domain()[1] - this.scale.x.domain()[0]) / 86400000 >
+          15
+        )
+          return [900, 3600].includes(difference);
+        return difference === 900;
+      };
     }
   },
   created() {