Add a note about None for 'marker'
@@ -64,7 +64,8 @@ class PlotReport(extension.Extension):
in this order. This callback can modify the figure.
file_name (str): Name of the figure file under the output directory.
It can be a format string.
- marker (str): The marker used to plot the graph. Default is 'x'
+ marker (str): The marker used to plot the graph. Default is ``'x'``. If
+ ``None`` is given, it draws with no markers.
grid (bool): Set the axis grid on if True. Default is True.
"""
