code: def get_stack(self, xvals, yvals, baselines, sign='positive'):
          
          bottoms, tops = [], []
          for x, y in zip(xvals, yvals):
              baseline = baselines[x][sign]
              if sign == 'positive':
                  bottom = baseline
                  top = bottom+y
                  baseline = top
              else:
                  top = baseline
                  bottom = top+y
                  baseline = bottom
              baselines[x][sign] = baseline
              bottoms.append(bottom)
              tops.append(top)
          return bottoms, tops
docs: Iterates over a x- and y-values in a stack layer
          and appropriately offsets the layer on top of the
          previous layer.