Easy Chart ignoring values

I have an easy chart that is tracking thermocouple data for parts in an oven. Frequently the parts are removed and the thermocouples are unplugged causing there values to go to 770. What i would like to do is have the easychart ignore all pen data at or above 770 for trending and scaling purposes.

I have tried the following:

a script to write the tags to null if the OPC tag is above 700, but the easychart plots the null values as 0.
let my script write to a string forcing the tags to null with a bad tag quality - this works except the x-trace shows the easy chart date = 12-31-1969 which breaks my chart annotations which watch that date property.
the documentation is insufficient to know if the easychart properties “where clause” could be used or if it is only able to be used for database tags

Any advice would be helpful

Thanks

It can only be used with database pens.