Interesting, this effect is usually seen when there is a mismatch between the post rate for data received in emoncms and that of the feed the data is recorded in. e.g the feed might have a 5s resolution but you are posting every 10s, the feed will then have a null value for every 2nd reading.
If you look at your battery SOC, battery charge and discharge feed in the graph module do you see missing data there as well?