What’s going on is basically this: your pen is set to “Average” mode. The easy chart has a fixed resolution, which means that for each query, it ends up with certain windows of time- and due to the setting, if multiple values exist for that window, they are averaged together. Depending on how things line up, values can shift between windows on subsequent queries, causing the result to be a bit different.
In these screen shots, the most noticeable point is the location where there is a gap. The last value on the edge of the gap is being averaged erroneously with 0 for the bad value, sometimes causing a dip of varying severity. This is a known issue that we hope to fix for 7.3.4.
You simplest option is to change the pen mode to “min/max”. This will create a thicker line, but it should be stable as the chart re-queries. Increasing the resolution of the chart could also possible help a bit (it would just make the jitters a bit less pronounced), but there would still be some shift. This setting is visible by selecting the “all” filter on the property pane for the chart.
Also, in 7.3 this isn’t such an issue due to caching on the client side. The chart doesn’t re-query the full range, so the previously calculated values don’t change.
Hope this helps clear things up.