use of data coordinates in axhline?

What the best way to to use data coordinates rather than axes coordinates for xmin and xmax with axhline?

Here's my kludgy solution, but it's not elegant:

ax = gca()
inv = ax.transAxes.inverted()
x = inv.transform(ax.transData.transform([10,0]))
ax.axhline(10, xmax=x[0])

I tried doing this:

ax.axhline(15, xmax=15, transform=ax.transData)

but I get:

ValueError: 'transform' is not allowed as a kwarg;axhline generates its own transform.

which is a bit strange since transform is listed in the kwargs in the help, though there is also this:

     Valid kwargs are :class:`~matplotlib.lines.Line2D` properties,
     with the exception of 'transform':

which could have two meanings: one is that all the kwargs listed are Line2D properties except 'transform' which is a valid kwarg, but not a Line2D property, OR even though 'transform' is listed, it's not a valid kwarg. Somewhat confusing especially given the ValueError...