I created a simple spreadsheet: column A=various dates, column B values matching those dates.

I then selected a simple line chart. I opted for the values to display at their respective chart points below the point. I then added a linear trendline.

The default trendline name is automatic vs. custom. Not understanding custom I left it as automatic. However, doing so then places _____Trendline (value) in the graph. Since 'properties' has the value grayed out with automatic I would have thought that Excel would insert a value for value (whatever that is?) since you can't, if if you knew what to insert, edit the (value) in the chart inserted label. If I were to select custom in properties the value box lights up for an entry--again, I don't have a clue what value would go in value since the result is simply a legend sort of thing in the chart?