Fix an indentation problem (probably from a merge) which left contour and vector plots only
partially set up.
This code would be better organized if the common min/max code were in a separate function, rather
than doing it with all these if/elifs.
Also I tweaked the calculation of the stride between plotted vectors.  It probably doesn't belong
here at all, but it's the best place available at the moment.  I'm not sure that the stride is
universally good, but at least the code is more transparent than before.