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I am trying to simulate and graph the knee voltage of a diode. I cannot seem to set up a current expression to measure on a DC sweep. I have tried "I(un1)" but it is red and appears to be invalid notation. How can I create a valid current expression? I must be missing something obvious.

I(un1) would be asking for the current at a node named "un1", but the simulator only knows about currents going into terminals of devices. If you mouse-over the terminals of a component, you'll be able to add an expression like "I(Q1.nB)" -- which is the current into Q1's terminal "nB" (or base in this example). These names are shown if you mouse over the endpoints. See also CircuitLab Documentation - Expressions.