Fix plotting multiple series with the same function

[TimoWilken]

If plot gets a function producing data, but multiple :using args, it currently reads all the output from the function on the first go, and subsequent :using args don't have anything left to plot.

Instead, save the data produced by the function for later, and output it every time.

This should fix #42.

I suppose a better way to do this would be to declare the data once in a gnuplot data block (as mentioned in #42) and then refer to it one or multiple times, but then eazy-gnuplot would require gnuplot >= 5.0.