clear canvas

raspberry/roberto/views/frontend/templates/index.html

@@ -174,6 +174,7 @@ socket.on('battery', (data) => {
   var ctx = canvas.getContext("2d");
   ctx.font = "14px Arial";
   ctx.fillStyle = "blue";
+  ctx.clearRect(3*canvas.width/4, 30, canvas.width/4, 24);
   ctx.fillText(data, 3*canvas.width/4, 30);
 });