docs: 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