To change the value range of a tag, modify the Plot Min and Max properties in the Tag Info Grid.
The value range can also be manually adjusted. Click and drag the Y-axis to pan up and down. This also works for individual tags when showing value ranges for all tags.
If your mouse has a scroll wheel, scroll up or down to narrow or widen the value ranges of all tags at once.
To automatically fit the Y-Axis to accommodate all tag values in the trend, use the Auto-Scale feature.