message: Doc review : Explain that plot persistence density function doesn't handle degenerate data
diff: @@ -249,7 +249,8 @@ def plot_persistence_density(persistence=[], persistence_file='',
  """This function plots the persistence density from persistence
  values list or from a :doc:`persistence file <fileformats>`. Be
  aware that this function does not distinguish the dimension, it is
  - up to you to select the required one.
  + up to you to select the required one. This function also does not handle
  + degenerate data set (scipy correlation matrix inversion can fail).
  :param persistence: Persistence intervals values list grouped by dimension.
  :type persistence: list of tuples(dimension, tuple(birth, death)).
  