class_docstr: Note: the Gantt charts listed here are inspired by
  Dustin Arendt and Svitlana Volkova. ESTEEM: A Novel Framework for Qualitatively Evaluating and
  Visualizing Spatiotemporal Embeddings in Social Media. ACL System Demonstrations. 2017.
  http://www.aclweb.org/anthology/P/P17/P17-4005.pdf
  
  In order to use the make chart function, Altair must be installed.
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code: def _get_negative_categories(self, cat, tdf):
      return sorted([x for x in tdf.columns if x < cat])[
          -self.timesteps_to_lag :
      ]
  
file_path: scattertext/scattertext/diachronic/GanttChart.py
imports: import numpy
  import pandas
  from scattertext.termranking import AbsoluteFrequencyRanker
  from scattertext.termscoring.RankDifference import RankDifference
label_desc: Use image node_id 4 for calling the GanttChart obj's underlying member method code with example usage: obj._get_negative_categories(cat, tdf) and returns: sorted