Trend chart settings aren't saved. The properties dialogue is opened by right-clicking on the open trend chart and then one or more of the settings are changed. After that OK is pressed and then the chart is saved using the save button.
TAC Vista 5.1.7.
It appears that the new settings aren't being saved, but by clicking the save button before refreshing the trend chart the current "view" is saved - not the new settings entered in the properties dialogue.
Change the settings as described (by clicking OK when finished with the settings) but refresh the trend chart immediately, then save it. In this way the view with the new settings will be saved.
Another way and noticed in Windows 7
Right click on the chart object.
Go to the "general" tab and uncheck Autoscale, enter the value you like.