gridlines at 0,0 only

  axvline(x=0, color='black', )

    > axhline(y=0, color='black', )

    > neither does this: i want a dashed line.

This is the right approach, just add linestyle='--' to the kwargs.
Since you will be using identical kwargs for the hline and vline, you
may want to consider the following approach, which is equivalent but
puts the configuration in one place

  props = dict(color='black', linestyle='--', linewidth=1)
  axvline(x=0, **props)
  axhline(y=0, **props)

If for some reason you *really* want to use the actual gridlines
functionality w/o affecting the ticks, you could selectively toggle
the visible property of the gridlines you do not want to show.