docs: def appendData(self, xdata, ydata, color='b', legendstr=None):
          """Adds the data to the plot
  
          :param xdata: index values for data, plotted on x-axis
          :type xdata: numpy.ndarray
          :param ydata: value data to plot, dimension must match xdata
          :type ydata: numpy.ndarray
          """
          item = self.plot(xdata, ydata, pen=color)
          if legendstr is not None:
              self.legend.addItem(item, legendstr)
          return item