Max,
I think Mache's advice is still the correct advice. See attached image of block diagram demonstrating how to programmatically modify the plot name (the one in the legend) and the plot line style of two plots on a graph. The technique here relies on the fact that property nodes are executed top-to-bottom in LabVIEW, so first the properties of Plot 0 are set, then the properties of Plot 1.
If you feed "Relative Humidity" to one of those Plot.Name inputs on the property node, you'll see that the plot legend updates accordingly.
Take care,
John