diff --git a/client/jsFiles/SEAWebClientGraphics.js b/client/jsFiles/SEAWebClientGraphics.js
index cbdaca4..78f428d 100644
--- a/client/jsFiles/SEAWebClientGraphics.js
+++ b/client/jsFiles/SEAWebClientGraphics.js
@@ -1940,10 +1940,15 @@ function Graph(gindex, container, x_label, y_label, tag, scaleType = "linear"){
         if (x === null) return;
         for(let i in chart.data.datasets){
             let y = null;
-            for(let j = 0; j < chart.getDatasetMeta(i).data.length; j++){
-                let dp = chart.getDatasetMeta(i).data[j];
-                if (dp.x >= x) break; //_model does not exist anymore, properties are defined directly on elements
-                y = chart.data.datasets[i].data[dp.$context.index].y; // $context not mentionned in ChartJS doc, seen with console.log
+            let metadata = chart.getDatasetMeta(i).data;
+            let dataset = chart.data.datasets[i];
+            if (metadata.length != dataset.data.length) {
+                console.log('length mismatch in dataset.data and metadata')
+            }
+            for(let j = 0; j < metadata.length; j++){
+                let dp = metadata[j];
+                if (dp.x >= x) break;
+                y = dataset.data[j].y;
             }
             valueElm = legendvalues[chart.data.datasets[i].key];
             if (labelMinWidth == 0) {
@@ -1952,7 +1957,9 @@ function Graph(gindex, container, x_label, y_label, tag, scaleType = "linear"){
                 labelMinWidth = valueElm.clientWidth;
                 valueElm.style.minWidth = labelMinWidth + 'px';
             }
-            if (y !== null) {
+            if (y == null) {
+                valueElm.innerHTML = '';
+            } else {
                 valueElm.innerHTML = strFormat(y, labelDigits);
             }
         }
diff --git a/seaweb.py b/seaweb.py
index 0bffd7b..b4b6396 100755
--- a/seaweb.py
+++ b/seaweb.py
@@ -57,6 +57,7 @@ def to_json_sse(msg):
 app = flask.Flask(__name__)
 
 update_rider = circularlog.Rider("upd")
+pollinterval = 0.1
 
 @app.route('/update')
 def get_update(path=None):
@@ -82,7 +83,7 @@ def get_update(path=None):
                     yield to_json_sse(msg)
                 if messages:
                     lastmsg = time.time()
-                    gevent.sleep(1)
+                    gevent.sleep(pollinterval)
                 else:
                     if time.time() > lastmsg + 30:
                         if not client.info():
@@ -91,7 +92,7 @@ def get_update(path=None):
                         yield to_json_sse(dict(type='heartbeat'))
                         lastmsg = time.time()
                     else:
-                        gevent.sleep(0.5)
+                        gevent.sleep(pollinterval)
         except (GeneratorExit, tcp_lineserver.Disconnected) as e:
             logging.info("except clause %r", repr(e))
             logging.info('CLOSED %s', client.id)