python3, matlibplot, gui, ValueError could not convert string to float

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/matplotlib/__init__.py", line 1814, in inner
return func(ax, *args, **kwargs)
label='_nolegend_'
self._x = float(xy[0])
ValueError: could not convert string to float: G1