Adding another identical expression with a dynamic line width - that would restrict me a lot though and I couldn't put in actual numbers the line should stretch to
Adding two new expressions (+- a dynamic number of initial expression) and filling in the area in between - but how would I do that?
Another idea: Stacked line chart, first expression is 0.9*expression, second expression is 0.2*expression, colour the first one transparent and the second one in a colour - this gives me the +-10% band of the expression. Any better solutions?