plot.xbal margin changes remain

[markmfredrickson]

When we set up margins for plots, we change the global margin values. These should be restored at the end of the function. This can cause issues if someone tries to plot multiple balance plots in a single PDF, for example.

[josherrickson]

This causes issues in Rstudio directly. Run several plot.xbal commands in a row, and the margins dance rightward.