matplotlib axvline truth ambiguous or list issue?

No, you can't pass a list to axvline. For multiple vertical lines within one line, something like this will do

[pylab.axvline(_x, linewidth=1, color='g') for _x in x]

Instead of multiple calls to axvline we can use the plot command itself but providing the correct transformation (this is many times faster if there are many lines):

import matplotlib.transforms as tx
ax = pylab.gca()
trans = tx.blended_transform_factory(ax.transData, ax.transAxes)
pylab.plot(np.repeat(x, 3), np.tile([.25, .75, np.nan], len(x)), linewidth=2, color='g', transform=trans)

axvline is for creating x vertical line.

Meaning at a certin x point from y-min to y-max.

x cannot be a list type.

a simple example:

axvline(x=.5, ymin=0.25, ymax=0.75)

You can read more here

If you want to create a rectangle you can use:

axvspan(xmin, xmax, ymin=0, ymax=1, **kwargs)

in your case xmin is 1 and x man is 300.

For completeness, there is also the possibility of using matplotlib.pyplots vlines. This function accepts a list of x-coordinates. Furthermore, you can specify where the lines need to start/end with the arguments ymin and ymax. In the case of this question the code would be:

import matplotlib.transforms as mt

fig, ax = plt.subplots()
ax.plot(list, values, label='Trend', color='k', linestyle='-')

trans = mt.blended_transform_factory(ax.transData, ax.transAxes)
ax.vlines(x, ymin=0, ymax=1, linewidth=1, color='g', transform=trans)

Using the transform argument makes it easier to have the lines from the top to the bottom of your plot. You can read more about it here. You can also skip that argument. In that case you have to specify ymin and ymax in actual y-coordinates.