used for building set of traces

def _init_trace(self, n,  color, style,
                    linewidth=2.5, zorder=None, marker=None, markersize=6):
        """ used for building set of traces"""
        while n >= len(self.traces):
            self.traces.append(LineProperties())
        line = self.traces[n]
        label = "trace %i" % (n+1)
        line.label = label
        line.drawstyle = 'default'
        if zorder     is None:
            zorder = 5 * (n+1)
        line.zorder = zorder
        if color      is not None: line.color = color
        if style      is not None: line.style = style
        if linewidth  is not None: line.linewidth = linewidth
        if marker     is not None: line.marker = marker
        if markersize is not None: line.markersize = markersize
        self.traces[n] = line