Calculate line width necessary to cover at least one pixel on all axes.

def _pixelsize(self, p):
        """Calculate line width necessary to cover at least one pixel on all axes."""
        xpixelsize = 1./float(p.xdensity)
        ypixelsize = 1./float(p.ydensity)
        return max([xpixelsize,ypixelsize])