Why would you need a legend if you just have 1 expression? The purpose of a legend is to let users easily distinguish which line/bar represents which expression. If you only have 1 expression, you can just make that the title of your chart...If you really need the expression to appear as a legend, you can create another expression with a constant that is very high (e.g. 999999), deselect "Expression as Legend" for the constant, and then set a static max in the axes tab to be below the constant (to hide the second expression).