By default, depending on the minimum and maximum tag values plotted in a trend
chart, the upper scale and lower scale of the y-axis are considered. You can, however,
choose to enter these scales manually.
Procedure
In the trend chart, select the drop-down list box that is labelled after the
tag name, and then select Edit.
The Chart Editor window appears.
For the tag selected in the Select Series field, switch
the Auto Scale toggle.
The Min and Max fields
appear.
Enter the lower scale and upper scale values in the Min
and Max field respectively, and then select
OK.
Historian
The Chart Editor window appears.