Making some minor changes to the plot_vec function to allow for flexible axis limits iff there are labels

[mdarmstr]

If there are labels, and there are negative values in the input to plot_vec, then the lables will often exceed the minimum ylim. I added an if statement in cases where labels are input, and there are negative values - in this case the y limits are based on +/- the maximum entry, to allow for long labels for negative values. Otherwise nothing changes.