This was going to be a question, but I solved it, so here's the answer too.
I have a line chart with two dimensions and three expressions, two of which are disabled until the first is down to one value.
I had Expressions as Legend turned on for all three expressions, but in the legend, while there were three labels present, only one of them had a line next to it - The first two were ambiguous because they didn't have a colored line next to them at all, so you couldn't tell them apart.
It turns out that it's because I had a Background Color expression on each of them that I'd changed my mind about, and rather than deleting it, I commented it out. For some reason, QlikView interprets a Background Color expression that's not blank but returns no value as white. When I deleted everything from the Background Color expression, the legend lines returned.