stipple - put markers on a plot to indicate value of interest
f=None - cf field or field
x=None - x points for field
y=None - y points for field
min=None - minimum threshold for stipple
max=None - maximum threshold for stipple
size=80 - default size for stipples
color=’k’ - default colour for stipples
pts=50 - number of points in the x direction
marker=’.’ - default marker for stipples
edegecolors=’k’ - outline colour
alpha=1.0 - transparency setting - default is off
ylog=False - set to True if a log pressure stipple plot
zorder=None - plotting order