The Highlight on Rollover feature highlights a trend line with the active plot by trebling it's line thickness. The highlighting is activated by moving the mouse cursor of the name of an item in the Item Details Area under the plot.
Note: By default, Highlight on Rollover is enabled. To disable this feature de-select it on the User Preferences Plot Tab.
Tip: Placing the mouse over the Item in the Item Details Area will also bring up the Item Properties tool-tip. To cancel the tool-tip whist maintaining the highlight, single left-click on the item name in the Item Details Area.