code: def derivative(xdata, ydata):
      
      D_ydata = []
      D_xdata = []
      for n in range(1, len(xdata)-1):
          D_xdata.append(xdata[n])
          D_ydata.append((ydata[n+1]-ydata[n-1])/(xdata[n+1]-xdata[n-1]))
  
      return [D_xdata, D_ydata]
docs: performs d(ydata)/d(xdata) with nearest-neighbor slopes
      must be well-ordered, returns new arrays [xdata, dydx_data]
  
      neighbors: