@gregoh This error was fixed in this file : https://github.com/mementum/backtrader/blob/master/backtrader/plot/locator.py
with PR #418

Before fix :

from matplotlib.dates import (HOURS_PER_DAY, MIN_PER_HOUR, SEC_PER_MIN, MONTHS_PER_YEAR, DAYS_PER_WEEK, SEC_PER_HOUR, SEC_PER_DAY, num2date, rrulewrapper, YearLocator, MicrosecondLocator, warnings)

After fix :

import warnings (...) from matplotlib.dates import (HOURS_PER_DAY, MIN_PER_HOUR, SEC_PER_MIN, MONTHS_PER_YEAR, DAYS_PER_WEEK, SEC_PER_HOUR, SEC_PER_DAY, num2date, rrulewrapper, YearLocator, MicrosecondLocator)

You can either install backtrader from its github sources or apply @sistemaon solution

To those that may come across this thread, I cannot explain why the issue is occurring, but if you pass a name to the default data when utilizing 'adddata', the legend will look normal again. Fixed

#Don't use this: cerebro.adddata(data) #Use a name with the data: cerebro.adddata(data, name='OHLCV_jt')

The name comes across in the heading / legend of the master Bokeh plot as so:
snapshot1.PNG

I tried it again with a pristine installation of backtrader (pip install backtrader). With the defaults, it's OK, but with voloverlay=False, the entire DataFrame is printed.
Imgur

@sobeasy It seems to me that your solution should work. But it blows up for me (Spyder 3.3.6, Anaconda3, Win10 64 bit). Somehow, it really wants tkinter!

ImportError: Cannot load backend 'TkAgg' which requires the 'tk' interactive framework, as 'qt5' is currently running

Time to try VS Code; PyCharm's footprint is too heavy.