There are a few things happening here.
First of all, you have the historian settings of the tag.
There’s the deadband, which determines when new values are saved. You can set it to an absolute value, or a relative value. Watch out, relative doesn’t mean relative to the current value, but relative to the engineering range (which is 0 to 100 by default). We don’t use engineering ranges for anything else, so we always use an absolute deadband as it’s easier to reason about.
My first guess is that you deadband is too big, so it doesn’t save new values.
Then you have the deadband style setting of “Analog” vs “Discrete”. This determines how the deadband is calculated. On an analog tag, it will extrapolate the value change, and only register a point if the measured value is outside the expected range (with respect to the deadband). In Discrete mode, it will register a value every time the next difference is more than the deadband. So if you have a perfectly linear evolution of the measurement, the analog tag will only register two measurements. While the discrete tag will register more points.
After that, the data needs to be queried. I believe (from expriments) the easy chart always queries 100 points in that time range (no matter how big your chart or your time range is). It’s very unlikely the requested time points will match exactly with measured time points. So the historian will interpolate between two measured points to find a good value. This again depends on the historian setting of “Analog” vs “Discrete”. An analog tag will get a linear interpollation, while a discrete tag will remain at the old value until a new value is read, and jump to that value vertically.
The current value of the tag is a special case here. The current value is always considered a data point, even if it doesn’t end up being stored (due to it being inside the deadband).
So in your case, the current value of 0 doesn’t get stored, but the historian does interpollate between the last stored value and the current value. Which is why you see the line moving.
Then you also have the settings of the easy chart. On the chart, you can choose the drawing style. You have the choice between a few “Line” and “Digital” pens. As the chart is often more than 100px wide, it also needs to interpollate between the points it gets back from the historian. When you set the style to one of the “Line” styles, it will interpollate in an analog way, if you set it to “Digital”, it will interpolate like discrete tags. So you should match the easy chart rendering with your tag configuration, otherwise you get jagged lines (digital interpolation on analog tags), or slanted drops instead of straight drops (line interpollation on discrete tags).
The deadband is the most important setting to determine when values are stored. Alter it to get enough data points for your use case, but not too much to not put stress on the database.
If you zoom in deep enough to the current time, you will always see this moving line, but it’s no problem if your historical measures are correct.