axvlines: boolean option to parallel_coordinates plot

Determines whether vertical lines will be added to parallel_coordinates plot. I was using parallel coordinates to make a seasonality chart - plotting years on top of each other (days/months of year on x-axis). With that many xticks, the vertical lines were overwhelming. It's possible to remove the lines after the fact, but without specific line identifiers, it isn't a straight forward process. However, preventing them from being added is a one-liner...