More stable mpl_date conversion, just needs a timezone. (see below).

pytz is a dependency of matplotlib, so this isn't adding bloat to
our optional dependency list.

Author: theengineear

plotly/matplotlylib/mpltools.py

@@ -7,8 +7,9 @@
 
 import math
 import warnings
-import datetime
 import matplotlib.dates
+import pytz
+
 
 def check_bar_match(old_bar, new_bar):
     """Check if two bars belong in the same collection (bar chart).
@@ -455,13 +456,15 @@ 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"):
+    """Convert matplotlib dates to formatted datestrings for plotly.
+
+    Info on mpl dates: http://matplotlib.org/api/dates_api.html
+
+    """
     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.utcfromtimestamp(epoch_time)
-                      for epoch_time in epoch_times]
+        date_times = matplotlib.dates.num2date(mpl_dates, tz=pytz.utc)
         time_strings = [date_time.strftime(format_string)
                         for date_time in date_times]
         if len(time_strings) > 1: