In [1]: from astropy import units as u

In [2]: import matplotlib.pyplot as plt

In [3]: fig = plt.figure()

In [4]: ax = fig.add_subplot(1,1,1)

In [5]: x = [1,2] * u.m

In [6]: y = [3,4] * u.m

In [7]: ax.plot(x,y)
Out[7]: [<matplotlib.lines.Line2D at 0x107f00048>]

In [8]: ax.scatter(x, y)
Out[8]: <matplotlib.collections.PathCollection at 0x107f00ba8>

In [9]: z = 4 * u.m

In [10]: ax.axhline(z)
---------------------------------------------------------------------------
UnitsError                                Traceback (most recent call last)
<ipython-input-10-39d10b51e434> in <module>()
----> 1 ax.axhline(z)

/Users/tom/miniconda3/envs/production35/lib/python3.5/site-packages/matplotlib/axes/_axes.py in axhline(self, y, xmin, xmax, **kwargs)
    744         self._process_unit_info(ydata=y, kwargs=kwargs)
    745         yy = self.convert_yunits(y)
--> 746         scaley = (yy < ymin) or (yy > ymax)
    747 
    748         trans = self.get_yaxis_transform(which='grid')

/Users/tom/miniconda3/envs/production35/lib/python3.5/site-packages/astropy/units/quantity.py in __array_prepare__(self, obj, context)
    344                                      "argument is not a quantity (unless the "
    345                                      "latter is all zero/infinity/nan)"
--> 346                                      .format(function.__name__))
    347             except TypeError:
    348                 # _can_have_arbitrary_unit failed: arg could not be compared

UnitsError: Can only apply 'less' function to dimensionless quantities when other argument is not a quantity (unless the latter is all zero/infinity/nan)