This is an incomplete fix, but a reasonable one for now.

Basically, mpl has its own datetime system, where it uses floating
point versions of the Gregorian calendar days. My attempt to use
ISO formatted date-time strings failed. Therefore, I'm actually
manually formatting the time-string based on epoch time after
converting from mpl_date -> epoch time. Whew!

Author: theengineear

plotly/matplotlylib/mpltools.py

@@ -7,6 +7,8 @@
 
 import math
 import warnings
+import datetime
+import matplotlib.dates
 
 def check_bar_match(old_bar, new_bar):
     """Check if two bars belong in the same collection (bar chart).
@@ -416,6 +418,17 @@ def prep_ticks(ax, index, ax_type, props):
         return dict()
     # get tick label formatting information
     formatter = axis.get_major_formatter().__class__.__name__
+    if ax_type == 'x' and formatter == 'DateFormatter':
+        axis_dict['type'] = 'date'
+        try:
+            axis_dict['tick0'] = mpl_dates_to_datestrings(axis_dict['tick0'])
+        except KeyError:
+            pass
+        finally:
+            axis_dict.pop('dtick', None)
+            axis_dict.pop('autotick', None)
+            axis_dict['range'] = mpl_dates_to_datestrings(props['xlim'])
+
     if formatter == 'LogFormatterMathtext':
         axis_dict['exponentformat'] = 'e'
     return axis_dict
@@ -442,6 +455,22 @@ def prep_xy_axis(ax, props, x_bounds, y_bounds):
     yaxis.update(prep_ticks(ax, 1, 'y', props))
     return xaxis, yaxis
 
+def mpl_dates_to_datestrings(mpl_dates, format_string="%Y-%m-%d %H:%M:%S"):
+    try:
+        # make sure we have a list
+        mpl_date_list = list(mpl_dates)
+        epoch_times = matplotlib.dates.num2epoch(mpl_date_list)
+        date_times = [datetime.datetime.fromtimestamp(epoch_time)
+                      for epoch_time in epoch_times]
+        time_strings = [date_time.strftime(format_string)
+                        for date_time in date_times]
+        if len(time_strings) > 1:
+            return time_strings
+        else:
+            return time_strings[0]
+    except TypeError:
+        return mpl_dates
+
 
 DASH_MAP = {
     '10,0': 'solid',

plotly/matplotlylib/renderer.py

@@ -54,6 +54,7 @@ def __init__(self):
         self.bar_containers = None
         self.current_bars = []
         self.axis_ct = 0
+        self.x_is_mpl_date = False
         self.mpl_x_bounds = (0, 1)
         self.mpl_y_bounds = (0, 1)
         self.msg = "Initialized PlotlyRenderer\n"
@@ -176,6 +177,10 @@ def open_axes(self, ax, props):
         self.plotly_fig['layout']['xaxis{0}'.format(self.axis_ct)] = xaxis
         self.plotly_fig['layout']['yaxis{0}'.format(self.axis_ct)] = yaxis
 
+        # let all subsequent dates be handled properly if required
+        if xaxis.get('type') == 'date':
+            self.x_is_mpl_date = True
+
     def close_axes(self, ax):
         """Close the axes object and clean up.
 
@@ -190,6 +195,7 @@ def close_axes(self, ax):
         """
         self.draw_bars(self.current_bars)
         self.msg += "  Closing axes\n"
+        self.x_is_mpl_date = False
 
     def draw_bars(self, bars):
 
@@ -357,6 +363,10 @@ def draw_marked_line(self, **props):
                 yaxis='y{0}'.format(self.axis_ct),
                 line=line,
                 marker=marker)
+            if self.x_is_mpl_date:
+                marked_line['x'] = mpltools.mpl_dates_to_datestrings(
+                    marked_line['x']
+                )
             self.plotly_fig['data'] += marked_line,
             self.msg += "    Heck yeah, I drew that line\n"
         else: